diff --git a/.babelrc.js b/.babelrc.js
index b6aa67e24f..a6942cad53 100644
--- a/.babelrc.js
+++ b/.babelrc.js
@@ -2,6 +2,7 @@ const { NODE_ENV } = process.env
 
 module.exports = {
   presets: [
+    '@babel/typescript',
     [
       '@babel/env',
       {
diff --git a/.eslintignore b/.eslintignore
index 7fb555f25d..469147f561 100644
--- a/.eslintignore
+++ b/.eslintignore
@@ -3,3 +3,5 @@
 **/server.js
 **/webpack.config*.js
 **/flow-typed/**
+# TODO: figure out a way to lint this using flow instead of typescript
+examples/todos-flow
\ No newline at end of file
diff --git a/.eslintrc.js b/.eslintrc.js
index de792fb8a9..4e01bf55a7 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -1,22 +1,34 @@
 module.exports = {
   extends: 'react-app',
 
+  parser: '@typescript-eslint/parser',
+
+  plugins: ['@typescript-eslint'],
+
   settings: {
     react: {
       version: '16.8'
+    },
+    'import/parsers': {
+      '@typescript-eslint/parser': ['.ts', '.tsx']
+    },
+    'import/resolver': {
+      // use <root>/tsconfig.json
+      typescript: {}
     }
   },
 
   rules: {
-    'jsx-a11y/href-no-hash': 'off'
-  },
-
-  overrides: [
-    {
-      files: 'test/**/*.js',
-      env: {
-        jest: true,
-      },
-    },
-  ],
+    'jsx-a11y/href-no-hash': 'off',
+    'no-unused-vars': 'off',
+    '@typescript-eslint/no-unused-vars': [
+      'error',
+      {
+        vars: 'all',
+        args: 'after-used',
+        ignoreRestSiblings: true,
+        argsIgnorePattern: '^_' // ignore unused variables whose name is '_'
+      }
+    ]
+  }
 }
diff --git a/.gitignore b/.gitignore
index cc152e1c4f..246860cfdc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,10 +1,11 @@
-.DS_Store
-*.log
 node_modules
+
+coverage
+
 dist
 lib
 es
-coverage
+types
 
 website/translated_docs
 website/build/
diff --git a/examples/todomvc/src/components/App.spec.js b/examples/todomvc/src/components/App.spec.js
index 2b6144e03d..ed51df865d 100644
--- a/examples/todomvc/src/components/App.spec.js
+++ b/examples/todomvc/src/components/App.spec.js
@@ -4,8 +4,7 @@ import App from './App'
 import Header from '../containers/Header'
 import MainSection from '../containers/MainSection'
 
-
-const setup = propOverrides => {
+const setup = _propOverrides => {
   const renderer = createRenderer()
   renderer.render(<App />)
   const output = renderer.getRenderOutput()
@@ -16,16 +15,16 @@ describe('components', () => {
   describe('Header', () => {
     it('should render', () => {
       const output = setup()
-      const [ header ] = output.props.children
+      const [header] = output.props.children
       expect(header.type).toBe(Header)
     })
   })
-  
+
   describe('Mainsection', () => {
     it('should render', () => {
       const output = setup()
-      const [ , mainSection ] = output.props.children
+      const [, mainSection] = output.props.children
       expect(mainSection.type).toBe(MainSection)
     })
   })
-})
\ No newline at end of file
+})
diff --git a/index.d.ts b/index.d.ts
index 50e8d018fa..aa8ec2ad31 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -211,7 +211,7 @@ export function combineReducers<M extends ReducersMapObject<any, any>>(
  *   dispatched.
  */
 export interface Dispatch<A extends Action = AnyAction> {
-  <T extends A>(action: T): T
+  <T extends A>(action: T, ...extraArgs: any[]): T
 }
 
 /**
@@ -247,6 +247,17 @@ export type Observer<T> = {
   next?(value: T): void
 }
 
+/**
+ * Extend the state
+ *
+ * This is used by store enhancers and store creators to extend state.
+ * If there is no state extension, it just returns the state, as is, otherwise
+ * it returns the state joined with its extension.
+ */
+export type ExtendState<State, Extension> = [Extension] extends [never]
+  ? State
+  : State & Extension
+
 /**
  * A store is an object that holds the application's state tree.
  * There should only be a single store in a Redux app, as the composition
@@ -254,8 +265,15 @@ export type Observer<T> = {
  *
  * @template S The type of state held by this store.
  * @template A the type of actions which may be dispatched by this store.
+ * @template StateExt any extension to state from store enhancers
+ * @template Ext any extensions to the store from store enhancers
  */
-export interface Store<S = any, A extends Action = AnyAction> {
+export interface Store<
+  S = any,
+  A extends Action = AnyAction,
+  StateExt = never,
+  Ext = {}
+> {
   /**
    * Dispatches an action. It is the only way to trigger a state change.
    *
@@ -326,7 +344,9 @@ export interface Store<S = any, A extends Action = AnyAction> {
    *
    * @param nextReducer The reducer for the store to use instead.
    */
-  replaceReducer(nextReducer: Reducer<S, A>): void
+  replaceReducer<NewState, NewActions extends Action>(
+    nextReducer: Reducer<NewState, NewActions>
+  ): Store<ExtendState<NewState, StateExt>, NewActions, StateExt, Ext> & Ext
 
   /**
    * Interoperability point for observable/reactive libraries.
@@ -353,15 +373,15 @@ export type DeepPartial<T> = {
  * @template StateExt State extension that is mixed into the state type.
  */
 export interface StoreCreator {
-  <S, A extends Action, Ext, StateExt>(
+  <S, A extends Action, Ext = {}, StateExt = never>(
     reducer: Reducer<S, A>,
     enhancer?: StoreEnhancer<Ext, StateExt>
-  ): Store<S & StateExt, A> & Ext
-  <S, A extends Action, Ext, StateExt>(
+  ): Store<ExtendState<S, StateExt>, A, StateExt, Ext> & Ext
+  <S, A extends Action, Ext = {}, StateExt = never>(
     reducer: Reducer<S, A>,
     preloadedState?: PreloadedState<S>,
     enhancer?: StoreEnhancer<Ext>
-  ): Store<S & StateExt, A> & Ext
+  ): Store<ExtendState<S, StateExt>, A, StateExt, Ext> & Ext
 }
 
 /**
@@ -415,16 +435,17 @@ export const createStore: StoreCreator
  * @template Ext Store extension that is mixed into the Store type.
  * @template StateExt State extension that is mixed into the state type.
  */
-export type StoreEnhancer<Ext = {}, StateExt = {}> = (
-  next: StoreEnhancerStoreCreator
+export type StoreEnhancer<Ext = {}, StateExt = never> = (
+  next: StoreEnhancerStoreCreator<Ext, StateExt>
 ) => StoreEnhancerStoreCreator<Ext, StateExt>
-export type StoreEnhancerStoreCreator<Ext = {}, StateExt = {}> = <
+
+export type StoreEnhancerStoreCreator<Ext = {}, StateExt = never> = <
   S = any,
   A extends Action = AnyAction
 >(
   reducer: Reducer<S, A>,
   preloadedState?: PreloadedState<S>
-) => Store<S & StateExt, A> & Ext
+) => Store<ExtendState<S, StateExt>, A, StateExt, Ext> & Ext
 
 /* middleware */
 
@@ -667,3 +688,9 @@ export function compose<R>(
 ): (...args: any[]) => R
 
 export function compose<R>(...funcs: Function[]): (...args: any[]) => R
+
+export const __DO_NOT_USE__ActionTypes: {
+  INIT: string
+  REPLACE: string
+  PROBE_UNKNOWN_ACTION: () => string
+}
diff --git a/package-lock.json b/package-lock.json
index 1a2d4a32c4..6a2db6a392 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -113,6 +113,120 @@
         "@babel/types": "^7.4.4"
       }
     },
+    "@babel/helper-create-class-features-plugin": {
+      "version": "7.5.5",
+      "resolved": "https://registry.npmjs.org/@babel/helper-create-class-features-plugin/-/helper-create-class-features-plugin-7.5.5.tgz",
+      "integrity": "sha512-ZsxkyYiRA7Bg+ZTRpPvB6AbOFKTFFK4LrvTet8lInm0V468MWCaSYJE+I7v2z2r8KNLtYiV+K5kTCnR7dvyZjg==",
+      "dev": true,
+      "requires": {
+        "@babel/helper-function-name": "^7.1.0",
+        "@babel/helper-member-expression-to-functions": "^7.5.5",
+        "@babel/helper-optimise-call-expression": "^7.0.0",
+        "@babel/helper-plugin-utils": "^7.0.0",
+        "@babel/helper-replace-supers": "^7.5.5",
+        "@babel/helper-split-export-declaration": "^7.4.4"
+      },
+      "dependencies": {
+        "@babel/code-frame": {
+          "version": "7.5.5",
+          "resolved": "https://registry.npmjs.org/@babel/code-frame/-/code-frame-7.5.5.tgz",
+          "integrity": "sha512-27d4lZoomVyo51VegxI20xZPuSHusqbQag/ztrBC7wegWoQ1nLREPVSKSW8byhTlzTKyNE4ifaTA6lCp7JjpFw==",
+          "dev": true,
+          "requires": {
+            "@babel/highlight": "^7.0.0"
+          }
+        },
+        "@babel/generator": {
+          "version": "7.5.5",
+          "resolved": "https://registry.npmjs.org/@babel/generator/-/generator-7.5.5.tgz",
+          "integrity": "sha512-ETI/4vyTSxTzGnU2c49XHv2zhExkv9JHLTwDAFz85kmcwuShvYG2H08FwgIguQf4JC75CBnXAUM5PqeF4fj0nQ==",
+          "dev": true,
+          "requires": {
+            "@babel/types": "^7.5.5",
+            "jsesc": "^2.5.1",
+            "lodash": "^4.17.13",
+            "source-map": "^0.5.0",
+            "trim-right": "^1.0.1"
+          }
+        },
+        "@babel/helper-member-expression-to-functions": {
+          "version": "7.5.5",
+          "resolved": "https://registry.npmjs.org/@babel/helper-member-expression-to-functions/-/helper-member-expression-to-functions-7.5.5.tgz",
+          "integrity": "sha512-5qZ3D1uMclSNqYcXqiHoA0meVdv+xUEex9em2fqMnrk/scphGlGgg66zjMrPJESPwrFJ6sbfFQYUSa0Mz7FabA==",
+          "dev": true,
+          "requires": {
+            "@babel/types": "^7.5.5"
+          }
+        },
+        "@babel/helper-replace-supers": {
+          "version": "7.5.5",
+          "resolved": "https://registry.npmjs.org/@babel/helper-replace-supers/-/helper-replace-supers-7.5.5.tgz",
+          "integrity": "sha512-XvRFWrNnlsow2u7jXDuH4jDDctkxbS7gXssrP4q2nUD606ukXHRvydj346wmNg+zAgpFx4MWf4+usfC93bElJg==",
+          "dev": true,
+          "requires": {
+            "@babel/helper-member-expression-to-functions": "^7.5.5",
+            "@babel/helper-optimise-call-expression": "^7.0.0",
+            "@babel/traverse": "^7.5.5",
+            "@babel/types": "^7.5.5"
+          }
+        },
+        "@babel/parser": {
+          "version": "7.5.5",
+          "resolved": "https://registry.npmjs.org/@babel/parser/-/parser-7.5.5.tgz",
+          "integrity": "sha512-E5BN68cqR7dhKan1SfqgPGhQ178bkVKpXTPEXnFJBrEt8/DKRZlybmy+IgYLTeN7tp1R5Ccmbm2rBk17sHYU3g==",
+          "dev": true
+        },
+        "@babel/traverse": {
+          "version": "7.5.5",
+          "resolved": "https://registry.npmjs.org/@babel/traverse/-/traverse-7.5.5.tgz",
+          "integrity": "sha512-MqB0782whsfffYfSjH4TM+LMjrJnhCNEDMDIjeTpl+ASaUvxcjoiVCo/sM1GhS1pHOXYfWVCYneLjMckuUxDaQ==",
+          "dev": true,
+          "requires": {
+            "@babel/code-frame": "^7.5.5",
+            "@babel/generator": "^7.5.5",
+            "@babel/helper-function-name": "^7.1.0",
+            "@babel/helper-split-export-declaration": "^7.4.4",
+            "@babel/parser": "^7.5.5",
+            "@babel/types": "^7.5.5",
+            "debug": "^4.1.0",
+            "globals": "^11.1.0",
+            "lodash": "^4.17.13"
+          }
+        },
+        "@babel/types": {
+          "version": "7.5.5",
+          "resolved": "https://registry.npmjs.org/@babel/types/-/types-7.5.5.tgz",
+          "integrity": "sha512-s63F9nJioLqOlW3UkyMd+BYhXt44YuaFm/VV0VwuteqjYwRrObkU7ra9pY4wAJR3oXi8hJrMcrcJdO/HH33vtw==",
+          "dev": true,
+          "requires": {
+            "esutils": "^2.0.2",
+            "lodash": "^4.17.13",
+            "to-fast-properties": "^2.0.0"
+          }
+        },
+        "debug": {
+          "version": "4.1.1",
+          "resolved": "https://registry.npmjs.org/debug/-/debug-4.1.1.tgz",
+          "integrity": "sha512-pYAIzeRo8J6KPEaJ0VWOh5Pzkbw/RetuzehGM7QRRX5he4fPHx2rdKMB256ehJCkX+XRQm16eZLqLNS8RSZXZw==",
+          "dev": true,
+          "requires": {
+            "ms": "^2.1.1"
+          }
+        },
+        "lodash": {
+          "version": "4.17.15",
+          "resolved": "https://registry.npmjs.org/lodash/-/lodash-4.17.15.tgz",
+          "integrity": "sha512-8xOcRHvCjnocdS5cpwXQXVzmmh5e5+saE2QGoeQmbKmRS6J3VQppPOIt0MnmE+4xlZoumy0GPG0D0MVIQbNA1A==",
+          "dev": true
+        },
+        "ms": {
+          "version": "2.1.2",
+          "resolved": "https://registry.npmjs.org/ms/-/ms-2.1.2.tgz",
+          "integrity": "sha512-sGkPx+VjMtmA6MX27oA4FBFELFCZZ4S4XqeGOXCv68tT+jb3vk/RyaKWP0PTKyWtmLSM0b+adUTEvbs1PEaH2w==",
+          "dev": true
+        }
+      }
+    },
     "@babel/helper-define-map": {
       "version": "7.4.4",
       "resolved": "https://registry.npmjs.org/@babel/helper-define-map/-/helper-define-map-7.4.4.tgz",
@@ -450,6 +564,15 @@
         "@babel/helper-plugin-utils": "^7.0.0"
       }
     },
+    "@babel/plugin-syntax-typescript": {
+      "version": "7.3.3",
+      "resolved": "https://registry.npmjs.org/@babel/plugin-syntax-typescript/-/plugin-syntax-typescript-7.3.3.tgz",
+      "integrity": "sha512-dGwbSMA1YhVS8+31CnPR7LB4pcbrzcV99wQzby4uAfrkZPYZlQ7ImwdpzLqi6Z6IL02b8IAL379CaMwo0x5Lag==",
+      "dev": true,
+      "requires": {
+        "@babel/helper-plugin-utils": "^7.0.0"
+      }
+    },
     "@babel/plugin-transform-arrow-functions": {
       "version": "7.2.0",
       "resolved": "https://registry.npmjs.org/@babel/plugin-transform-arrow-functions/-/plugin-transform-arrow-functions-7.2.0.tgz",
@@ -757,6 +880,17 @@
         "@babel/helper-plugin-utils": "^7.0.0"
       }
     },
+    "@babel/plugin-transform-typescript": {
+      "version": "7.5.5",
+      "resolved": "https://registry.npmjs.org/@babel/plugin-transform-typescript/-/plugin-transform-typescript-7.5.5.tgz",
+      "integrity": "sha512-pehKf4m640myZu5B2ZviLaiBlxMCjSZ1qTEO459AXKX5GnPueyulJeCqZFs1nz/Ya2dDzXQ1NxZ/kKNWyD4h6w==",
+      "dev": true,
+      "requires": {
+        "@babel/helper-create-class-features-plugin": "^7.5.5",
+        "@babel/helper-plugin-utils": "^7.0.0",
+        "@babel/plugin-syntax-typescript": "^7.2.0"
+      }
+    },
     "@babel/plugin-transform-unicode-regex": {
       "version": "7.4.4",
       "resolved": "https://registry.npmjs.org/@babel/plugin-transform-unicode-regex/-/plugin-transform-unicode-regex-7.4.4.tgz",
@@ -846,6 +980,16 @@
         "@babel/plugin-transform-flow-strip-types": "^7.0.0"
       }
     },
+    "@babel/preset-typescript": {
+      "version": "7.3.3",
+      "resolved": "https://registry.npmjs.org/@babel/preset-typescript/-/preset-typescript-7.3.3.tgz",
+      "integrity": "sha512-mzMVuIP4lqtn4du2ynEfdO0+RYcslwrZiJHXu4MGaC1ctJiW2fyaeDrtjJGs7R/KebZ1sgowcIoWf4uRpEfKEg==",
+      "dev": true,
+      "requires": {
+        "@babel/helper-plugin-utils": "^7.0.0",
+        "@babel/plugin-transform-typescript": "^7.3.2"
+      }
+    },
     "@babel/register": {
       "version": "7.4.4",
       "resolved": "https://registry.npmjs.org/@babel/register/-/register-7.4.4.tgz",
@@ -1236,6 +1380,33 @@
         "@types/istanbul-lib-report": "*"
       }
     },
+    "@types/jest": {
+      "version": "24.0.17",
+      "resolved": "https://registry.npmjs.org/@types/jest/-/jest-24.0.17.tgz",
+      "integrity": "sha512-1cy3xkOAfSYn78dsBWy4M3h/QF/HeWPchNFDjysVtp3GHeTdSmtluNnELfCmfNRRHo0OWEcpf+NsEJQvwQfdqQ==",
+      "dev": true,
+      "requires": {
+        "@types/jest-diff": "*"
+      }
+    },
+    "@types/jest-diff": {
+      "version": "20.0.1",
+      "resolved": "https://registry.npmjs.org/@types/jest-diff/-/jest-diff-20.0.1.tgz",
+      "integrity": "sha512-yALhelO3i0hqZwhjtcr6dYyaLoCHbAMshwtj6cGxTvHZAKXHsYGdff6E8EPw3xLKY0ELUTQ69Q1rQiJENnccMA==",
+      "dev": true
+    },
+    "@types/json-schema": {
+      "version": "7.0.3",
+      "resolved": "https://registry.npmjs.org/@types/json-schema/-/json-schema-7.0.3.tgz",
+      "integrity": "sha512-Il2DtDVRGDcqjDtE+rF8iqg1CArehSK84HZJCT7AMITlyXRBpuPhqGLDQMowraqqu1coEaimg4ZOqggt6L6L+A==",
+      "dev": true
+    },
+    "@types/json5": {
+      "version": "0.0.29",
+      "resolved": "https://registry.npmjs.org/@types/json5/-/json5-0.0.29.tgz",
+      "integrity": "sha1-7ihweulOEdK4J7y+UnC86n8+ce4=",
+      "dev": true
+    },
     "@types/node": {
       "version": "12.6.1",
       "resolved": "https://registry.npmjs.org/@types/node/-/node-12.6.1.tgz",
@@ -1264,12 +1435,12 @@
       "dev": true
     },
     "@typescript-eslint/eslint-plugin": {
-      "version": "1.11.0",
-      "resolved": "https://registry.npmjs.org/@typescript-eslint/eslint-plugin/-/eslint-plugin-1.11.0.tgz",
-      "integrity": "sha512-mXv9ccCou89C8/4avKHuPB2WkSZyY/XcTQUXd5LFZAcLw1I3mWYVjUu6eS9Ja0QkP/ClolbcW9tb3Ov/pMdcqw==",
+      "version": "1.13.0",
+      "resolved": "https://registry.npmjs.org/@typescript-eslint/eslint-plugin/-/eslint-plugin-1.13.0.tgz",
+      "integrity": "sha512-WQHCozMnuNADiqMtsNzp96FNox5sOVpU8Xt4meaT4em8lOG1SrOv92/mUbEHQVh90sldKSfcOc/I0FOb/14G1g==",
       "dev": true,
       "requires": {
-        "@typescript-eslint/experimental-utils": "1.11.0",
+        "@typescript-eslint/experimental-utils": "1.13.0",
         "eslint-utils": "^1.3.1",
         "functional-red-black-tree": "^1.0.1",
         "regexpp": "^2.0.1",
@@ -1277,12 +1448,13 @@
       }
     },
     "@typescript-eslint/experimental-utils": {
-      "version": "1.11.0",
-      "resolved": "https://registry.npmjs.org/@typescript-eslint/experimental-utils/-/experimental-utils-1.11.0.tgz",
-      "integrity": "sha512-7LbfaqF6B8oa8cp/315zxKk8FFzosRzzhF8Kn/ZRsRsnpm7Qcu25cR/9RnAQo5utZ2KIWVgaALr+ZmcbG47ruw==",
+      "version": "1.13.0",
+      "resolved": "https://registry.npmjs.org/@typescript-eslint/experimental-utils/-/experimental-utils-1.13.0.tgz",
+      "integrity": "sha512-zmpS6SyqG4ZF64ffaJ6uah6tWWWgZ8m+c54XXgwFtUv0jNz8aJAVx8chMCvnk7yl6xwn8d+d96+tWp7fXzTuDg==",
       "dev": true,
       "requires": {
-        "@typescript-eslint/typescript-estree": "1.11.0",
+        "@types/json-schema": "^7.0.3",
+        "@typescript-eslint/typescript-estree": "1.13.0",
         "eslint-scope": "^4.0.0"
       },
       "dependencies": {
@@ -1299,21 +1471,21 @@
       }
     },
     "@typescript-eslint/parser": {
-      "version": "1.11.0",
-      "resolved": "https://registry.npmjs.org/@typescript-eslint/parser/-/parser-1.11.0.tgz",
-      "integrity": "sha512-5xBExyXaxVyczrZvbRKEXvaTUFFq7gIM9BynXukXZE0zF3IQP/FxF4mPmmh3gJ9egafZFqByCpPTFm3dk4SY7Q==",
+      "version": "1.13.0",
+      "resolved": "https://registry.npmjs.org/@typescript-eslint/parser/-/parser-1.13.0.tgz",
+      "integrity": "sha512-ITMBs52PCPgLb2nGPoeT4iU3HdQZHcPaZVw+7CsFagRJHUhyeTgorEwHXhFf3e7Evzi8oujKNpHc8TONth8AdQ==",
       "dev": true,
       "requires": {
         "@types/eslint-visitor-keys": "^1.0.0",
-        "@typescript-eslint/experimental-utils": "1.11.0",
-        "@typescript-eslint/typescript-estree": "1.11.0",
+        "@typescript-eslint/experimental-utils": "1.13.0",
+        "@typescript-eslint/typescript-estree": "1.13.0",
         "eslint-visitor-keys": "^1.0.0"
       }
     },
     "@typescript-eslint/typescript-estree": {
-      "version": "1.11.0",
-      "resolved": "https://registry.npmjs.org/@typescript-eslint/typescript-estree/-/typescript-estree-1.11.0.tgz",
-      "integrity": "sha512-fquUHF5tAx1sM2OeRCC7wVxFd1iMELWMGCzOSmJ3pLzArj9+kRixdlC4d5MncuzXpjEqc6045p3KwM0o/3FuUA==",
+      "version": "1.13.0",
+      "resolved": "https://registry.npmjs.org/@typescript-eslint/typescript-estree/-/typescript-estree-1.13.0.tgz",
+      "integrity": "sha512-b5rCmd2e6DCC6tCTN9GSUAuxdYwCM/k/2wdjHGrIRGPSJotWMCe/dGpi66u42bhuh8q3QBzqM4TMA1GUUCJvdw==",
       "dev": true,
       "requires": {
         "lodash.unescape": "4.0.1",
@@ -2227,6 +2399,12 @@
       "integrity": "sha1-s2nW+128E+7PUk+RsHD+7cNXzzQ=",
       "dev": true
     },
+    "deepmerge": {
+      "version": "2.2.1",
+      "resolved": "https://registry.npmjs.org/deepmerge/-/deepmerge-2.2.1.tgz",
+      "integrity": "sha512-R9hc1Xa/NOBi9WRVUWg19rl1UB7Tt4kuPd+thNJgFZoxXsTz7ncaPaeIm+40oSGuP33DfMb4sZt1QIGiJzC4EA==",
+      "dev": true
+    },
     "define-properties": {
       "version": "1.1.3",
       "resolved": "https://registry.npmjs.org/define-properties/-/define-properties-1.1.3.tgz",
@@ -2524,10 +2702,38 @@
         "resolve": "^1.5.0"
       }
     },
+    "eslint-import-resolver-typescript": {
+      "version": "1.1.1",
+      "resolved": "https://registry.npmjs.org/eslint-import-resolver-typescript/-/eslint-import-resolver-typescript-1.1.1.tgz",
+      "integrity": "sha512-jqSfumQ+H5y3FUJ6NjRkbOQSUOlbBucGTN3ELymOtcDBbPjVdm/luvJuCfCaIXGh8sEF26ma1qVdtDgl9ndhUg==",
+      "dev": true,
+      "requires": {
+        "debug": "^4.0.1",
+        "resolve": "^1.4.0",
+        "tsconfig-paths": "^3.6.0"
+      },
+      "dependencies": {
+        "debug": {
+          "version": "4.1.1",
+          "resolved": "https://registry.npmjs.org/debug/-/debug-4.1.1.tgz",
+          "integrity": "sha512-pYAIzeRo8J6KPEaJ0VWOh5Pzkbw/RetuzehGM7QRRX5he4fPHx2rdKMB256ehJCkX+XRQm16eZLqLNS8RSZXZw==",
+          "dev": true,
+          "requires": {
+            "ms": "^2.1.1"
+          }
+        },
+        "ms": {
+          "version": "2.1.2",
+          "resolved": "https://registry.npmjs.org/ms/-/ms-2.1.2.tgz",
+          "integrity": "sha512-sGkPx+VjMtmA6MX27oA4FBFELFCZZ4S4XqeGOXCv68tT+jb3vk/RyaKWP0PTKyWtmLSM0b+adUTEvbs1PEaH2w==",
+          "dev": true
+        }
+      }
+    },
     "eslint-module-utils": {
-      "version": "2.4.0",
-      "resolved": "https://registry.npmjs.org/eslint-module-utils/-/eslint-module-utils-2.4.0.tgz",
-      "integrity": "sha512-14tltLm38Eu3zS+mt0KvILC3q8jyIAH518MlG+HO0p+yK885Lb1UHTY/UgR91eOyGdmxAPb+OLoW4znqIT6Ndw==",
+      "version": "2.4.1",
+      "resolved": "https://registry.npmjs.org/eslint-module-utils/-/eslint-module-utils-2.4.1.tgz",
+      "integrity": "sha512-H6DOj+ejw7Tesdgbfs4jeS4YMFrT8uI8xwd1gtQqXssaR0EQ26L+2O/w6wkYFy2MymON0fTwHmXBvvfLNZVZEw==",
       "dev": true,
       "requires": {
         "debug": "^2.6.8",
@@ -2598,9 +2804,9 @@
       }
     },
     "eslint-plugin-import": {
-      "version": "2.18.0",
-      "resolved": "https://registry.npmjs.org/eslint-plugin-import/-/eslint-plugin-import-2.18.0.tgz",
-      "integrity": "sha512-PZpAEC4gj/6DEMMoU2Df01C5c50r7zdGIN52Yfi7CvvWaYssG7Jt5R9nFG5gmqodxNOz9vQS87xk6Izdtpdrig==",
+      "version": "2.18.2",
+      "resolved": "https://registry.npmjs.org/eslint-plugin-import/-/eslint-plugin-import-2.18.2.tgz",
+      "integrity": "sha512-5ohpsHAiUBRNaBWAF08izwUGlbrJoJJ+W9/TBwsGoR1MnlgfwMIKrFeSjWbt6moabiXW9xNvtFz+97KHRfI4HQ==",
       "dev": true,
       "requires": {
         "array-includes": "^3.0.3",
@@ -2610,8 +2816,8 @@
         "eslint-import-resolver-node": "^0.3.2",
         "eslint-module-utils": "^2.4.0",
         "has": "^1.0.3",
-        "lodash": "^4.17.11",
         "minimatch": "^3.0.4",
+        "object.values": "^1.1.0",
         "read-pkg-up": "^2.0.0",
         "resolve": "^1.11.0"
       },
@@ -3172,6 +3378,17 @@
         "map-cache": "^0.2.2"
       }
     },
+    "fs-extra": {
+      "version": "8.1.0",
+      "resolved": "https://registry.npmjs.org/fs-extra/-/fs-extra-8.1.0.tgz",
+      "integrity": "sha512-yhlQgA6mnOJUKOsRUFsgJdQCvkKhcz8tlZG5HBQfReYZy46OwLcY+Zia0mtdHsOo9y/hP+CxMN0TU9QxoOtG4g==",
+      "dev": true,
+      "requires": {
+        "graceful-fs": "^4.2.0",
+        "jsonfile": "^4.0.0",
+        "universalify": "^0.1.0"
+      }
+    },
     "fs-readdir-recursive": {
       "version": "1.1.0",
       "resolved": "https://registry.npmjs.org/fs-readdir-recursive/-/fs-readdir-recursive-1.1.0.tgz",
@@ -4958,6 +5175,15 @@
         }
       }
     },
+    "jsonfile": {
+      "version": "4.0.0",
+      "resolved": "https://registry.npmjs.org/jsonfile/-/jsonfile-4.0.0.tgz",
+      "integrity": "sha1-h3Gq4HmbZAdrdmQPygWPnBDjPss=",
+      "dev": true,
+      "requires": {
+        "graceful-fs": "^4.1.6"
+      }
+    },
     "jsprim": {
       "version": "1.4.1",
       "resolved": "https://registry.npmjs.org/jsprim/-/jsprim-1.4.1.tgz",
@@ -6576,6 +6802,99 @@
         "terser": "^4.1.0"
       }
     },
+    "rollup-plugin-typescript2": {
+      "version": "0.24.0",
+      "resolved": "https://registry.npmjs.org/rollup-plugin-typescript2/-/rollup-plugin-typescript2-0.24.0.tgz",
+      "integrity": "sha512-yp7z9hZ5kXjOGXKXkfTRJuIPyLvKtUWfAGmreilnYn+qIVhE5CgE7SFvzLKHggsAXPaCAflHRiEj0l71WXWJkA==",
+      "dev": true,
+      "requires": {
+        "find-cache-dir": "^3.0.0",
+        "fs-extra": "8.1.0",
+        "resolve": "1.12.0",
+        "rollup-pluginutils": "2.8.1",
+        "tslib": "1.10.0"
+      },
+      "dependencies": {
+        "find-cache-dir": {
+          "version": "3.0.0",
+          "resolved": "https://registry.npmjs.org/find-cache-dir/-/find-cache-dir-3.0.0.tgz",
+          "integrity": "sha512-t7ulV1fmbxh5G9l/492O1p5+EBbr3uwpt6odhFTMc+nWyhmbloe+ja9BZ8pIBtqFWhOmCWVjx+pTW4zDkFoclw==",
+          "dev": true,
+          "requires": {
+            "commondir": "^1.0.1",
+            "make-dir": "^3.0.0",
+            "pkg-dir": "^4.1.0"
+          }
+        },
+        "find-up": {
+          "version": "4.1.0",
+          "resolved": "https://registry.npmjs.org/find-up/-/find-up-4.1.0.tgz",
+          "integrity": "sha512-PpOwAdQ/YlXQ2vj8a3h8IipDuYRi3wceVQQGYWxNINccq40Anw7BlsEXCMbt1Zt+OLA6Fq9suIpIWD0OsnISlw==",
+          "dev": true,
+          "requires": {
+            "locate-path": "^5.0.0",
+            "path-exists": "^4.0.0"
+          }
+        },
+        "locate-path": {
+          "version": "5.0.0",
+          "resolved": "https://registry.npmjs.org/locate-path/-/locate-path-5.0.0.tgz",
+          "integrity": "sha512-t7hw9pI+WvuwNJXwk5zVHpyhIqzg2qTlklJOf0mVxGSbe3Fp2VieZcduNYjaLDoy6p9uGpQEGWG87WpMKlNq8g==",
+          "dev": true,
+          "requires": {
+            "p-locate": "^4.1.0"
+          }
+        },
+        "make-dir": {
+          "version": "3.0.0",
+          "resolved": "https://registry.npmjs.org/make-dir/-/make-dir-3.0.0.tgz",
+          "integrity": "sha512-grNJDhb8b1Jm1qeqW5R/O63wUo4UXo2v2HMic6YT9i/HBlF93S8jkMgH7yugvY9ABDShH4VZMn8I+U8+fCNegw==",
+          "dev": true,
+          "requires": {
+            "semver": "^6.0.0"
+          }
+        },
+        "p-locate": {
+          "version": "4.1.0",
+          "resolved": "https://registry.npmjs.org/p-locate/-/p-locate-4.1.0.tgz",
+          "integrity": "sha512-R79ZZ/0wAxKGu3oYMlz8jy/kbhsNrS7SKZ7PxEHBgJ5+F2mtFW2fK2cOtBh1cHYkQsbzFV7I+EoRKe6Yt0oK7A==",
+          "dev": true,
+          "requires": {
+            "p-limit": "^2.2.0"
+          }
+        },
+        "path-exists": {
+          "version": "4.0.0",
+          "resolved": "https://registry.npmjs.org/path-exists/-/path-exists-4.0.0.tgz",
+          "integrity": "sha512-ak9Qy5Q7jYb2Wwcey5Fpvg2KoAc/ZIhLSLOSBmRmygPsGwkVVt0fZa0qrtMz+m6tJTAHfZQ8FnmB4MG4LWy7/w==",
+          "dev": true
+        },
+        "pkg-dir": {
+          "version": "4.2.0",
+          "resolved": "https://registry.npmjs.org/pkg-dir/-/pkg-dir-4.2.0.tgz",
+          "integrity": "sha512-HRDzbaKjC+AOWVXxAU/x54COGeIv9eb+6CkDSQoNTt4XyWoIJvuPsXizxu/Fr23EiekbtZwmh1IcIG/l/a10GQ==",
+          "dev": true,
+          "requires": {
+            "find-up": "^4.0.0"
+          }
+        },
+        "resolve": {
+          "version": "1.12.0",
+          "resolved": "https://registry.npmjs.org/resolve/-/resolve-1.12.0.tgz",
+          "integrity": "sha512-B/dOmuoAik5bKcD6s6nXDCjzUKnaDvdkRyAk6rsmsKLipWj4797iothd7jmmUhWTfinVMU+wc56rYKsit2Qy4w==",
+          "dev": true,
+          "requires": {
+            "path-parse": "^1.0.6"
+          }
+        },
+        "semver": {
+          "version": "6.3.0",
+          "resolved": "https://registry.npmjs.org/semver/-/semver-6.3.0.tgz",
+          "integrity": "sha512-b39TBaTSfV6yBrapU89p5fKekE2m/NwnDocOVruQFS1/veMgdzuPcnOM34M6CwxW8jH/lxEa5rBoDeUwu5HHTw==",
+          "dev": true
+        }
+      }
+    },
     "rollup-pluginutils": {
       "version": "2.8.1",
       "resolved": "https://registry.npmjs.org/rollup-pluginutils/-/rollup-pluginutils-2.8.1.tgz",
@@ -7265,6 +7584,36 @@
       "integrity": "sha1-yy4SAwZ+DI3h9hQJS5/kVwTqYAM=",
       "dev": true
     },
+    "tsconfig-paths": {
+      "version": "3.8.0",
+      "resolved": "https://registry.npmjs.org/tsconfig-paths/-/tsconfig-paths-3.8.0.tgz",
+      "integrity": "sha512-zZEYFo4sjORK8W58ENkRn9s+HmQFkkwydDG7My5s/fnfr2YYCaiyXe/HBUcIgU8epEKOXwiahOO+KZYjiXlWyQ==",
+      "dev": true,
+      "requires": {
+        "@types/json5": "^0.0.29",
+        "deepmerge": "^2.0.1",
+        "json5": "^1.0.1",
+        "minimist": "^1.2.0",
+        "strip-bom": "^3.0.0"
+      },
+      "dependencies": {
+        "json5": {
+          "version": "1.0.1",
+          "resolved": "https://registry.npmjs.org/json5/-/json5-1.0.1.tgz",
+          "integrity": "sha512-aKS4WQjPenRxiQsC93MNfjx+nbF4PAdYzmd/1JIj8HYzqfbu86beTuNgXDzPknWk0n0uARlyewZo4s++ES36Ow==",
+          "dev": true,
+          "requires": {
+            "minimist": "^1.2.0"
+          }
+        },
+        "minimist": {
+          "version": "1.2.0",
+          "resolved": "https://registry.npmjs.org/minimist/-/minimist-1.2.0.tgz",
+          "integrity": "sha1-o1AIsg9BOD7sH7kU9M1d95omQoQ=",
+          "dev": true
+        }
+      }
+    },
     "tslib": {
       "version": "1.10.0",
       "resolved": "https://registry.npmjs.org/tslib/-/tslib-1.10.0.tgz",
@@ -7272,9 +7621,9 @@
       "dev": true
     },
     "tsutils": {
-      "version": "3.14.0",
-      "resolved": "https://registry.npmjs.org/tsutils/-/tsutils-3.14.0.tgz",
-      "integrity": "sha512-SmzGbB0l+8I0QwsPgjooFRaRvHLBLNYM8SeQ0k6rtNDru5sCGeLJcZdwilNndN+GysuFjF5EIYgN8GfFG6UeUw==",
+      "version": "3.17.1",
+      "resolved": "https://registry.npmjs.org/tsutils/-/tsutils-3.17.1.tgz",
+      "integrity": "sha512-kzeQ5B8H3w60nFY2g8cJIuH7JDpsALXySGtwGJ0p2LSjLgay3NdIpqq5SoOBe46bKDW2iq25irHCr8wjomUS2g==",
       "dev": true,
       "requires": {
         "tslib": "^1.8.1"
@@ -7417,6 +7766,12 @@
         }
       }
     },
+    "universalify": {
+      "version": "0.1.2",
+      "resolved": "https://registry.npmjs.org/universalify/-/universalify-0.1.2.tgz",
+      "integrity": "sha512-rBJeI5CXAlmy1pV+617WB9J63U6XcazHHF2f2dbJix4XzpUF0RS3Zbj0FGIOCAva5P/d/GBOYaACQ1w+0azUkg==",
+      "dev": true
+    },
     "unset-value": {
       "version": "1.0.0",
       "resolved": "https://registry.npmjs.org/unset-value/-/unset-value-1.0.0.tgz",
diff --git a/package.json b/package.json
index 6dc69e8d77..772f99d6b5 100644
--- a/package.json
+++ b/package.json
@@ -26,26 +26,27 @@
   "main": "lib/redux.js",
   "unpkg": "dist/redux.js",
   "module": "es/redux.js",
-  "typings": "./index.d.ts",
+  "types": "types/index.d.ts",
   "files": [
     "dist",
     "lib",
     "es",
     "src",
-    "index.d.ts"
+    "types"
   ],
   "scripts": {
-    "clean": "rimraf lib dist es coverage",
+    "clean": "rimraf lib dist es coverage types",
     "format": "prettier --write \"{src,test}/**/*.{js,ts}\" index.d.ts \"**/*.md\"",
     "format:check": "prettier --list-different \"{src,test}/**/*.{js,ts}\" index.d.ts \"**/*.md\"",
-    "lint": "eslint src test",
+    "lint": "eslint --ext js,ts src test",
+    "check-types": "tsc --noEmit",
     "pretest": "npm run build",
     "test": "jest",
     "test:watch": "npm test -- --watch",
     "test:cov": "npm test -- --coverage",
     "build": "rollup -c",
-    "prepare": "npm run clean && npm run format:check && npm run lint && npm test",
-    "examples:lint": "eslint examples",
+    "prepare": "npm run clean && npm run check-types && npm run format:check && npm run lint && npm test",
+    "examples:lint": "eslint --ext js,ts examples",
     "examples:test": "cross-env CI=true babel-node examples/testAll.js"
   },
   "dependencies": {
@@ -60,15 +61,18 @@
     "@babel/plugin-proposal-object-rest-spread": "^7.5.4",
     "@babel/preset-env": "^7.5.4",
     "@babel/preset-flow": "^7.0.0",
+    "@babel/preset-typescript": "^7.3.3",
     "@babel/register": "^7.4.4",
-    "@typescript-eslint/eslint-plugin": "^1.11.0",
-    "@typescript-eslint/parser": "^1.11.0",
+    "@types/jest": "^24.0.17",
+    "@typescript-eslint/eslint-plugin": "^1.13.0",
+    "@typescript-eslint/parser": "^1.13.0",
     "babel-core": "^7.0.0-bridge.0",
     "babel-eslint": "^10.0.2",
     "babel-jest": "^24.8.0",
     "cross-env": "^5.2.0",
     "eslint": "^5.16.0",
     "eslint-config-react-app": "^4.0.1",
+    "eslint-import-resolver-typescript": "^1.1.1",
     "eslint-plugin-flowtype": "^2.50.3",
     "eslint-plugin-import": "^2.18.0",
     "eslint-plugin-jsx-a11y": "^6.2.3",
@@ -83,6 +87,7 @@
     "rollup-plugin-node-resolve": "^5.2.0",
     "rollup-plugin-replace": "^2.2.0",
     "rollup-plugin-terser": "^5.1.1",
+    "rollup-plugin-typescript2": "^0.24.0",
     "rxjs": "^6.5.2",
     "typescript": "^3.5.3",
     "typings-tester": "^0.3.2"
@@ -102,7 +107,7 @@
     ]
   },
   "jest": {
-    "testRegex": "(/test/.*\\.spec\\.js)$"
+    "testRegex": "(/test/.*\\.spec\\.[tj]s)$"
   },
   "sideEffects": false
 }
diff --git a/rollup.config.js b/rollup.config.js
index 07297f2bc2..27ce0b7bc9 100644
--- a/rollup.config.js
+++ b/rollup.config.js
@@ -1,42 +1,63 @@
 import nodeResolve from 'rollup-plugin-node-resolve'
 import babel from 'rollup-plugin-babel'
 import replace from 'rollup-plugin-replace'
+import typescript from 'rollup-plugin-typescript2'
 import { terser } from 'rollup-plugin-terser'
 
 import pkg from './package.json'
 
+const noDeclarationFiles = { compilerOptions: { declaration: false } };
+
 export default [
   // CommonJS
   {
-    input: 'src/index.js',
+    input: 'src/index.ts',
     output: { file: 'lib/redux.js', format: 'cjs', indent: false },
     external: [
       ...Object.keys(pkg.dependencies || {}),
       ...Object.keys(pkg.peerDependencies || {})
     ],
-    plugins: [babel()]
+    plugins: [
+      nodeResolve({
+        extensions: ['.ts']
+      }),
+      typescript({ useTsconfigDeclarationDir: true }),
+      babel()
+    ]
   },
 
   // ES
   {
-    input: 'src/index.js',
+    input: 'src/index.ts',
     output: { file: 'es/redux.js', format: 'es', indent: false },
     external: [
       ...Object.keys(pkg.dependencies || {}),
       ...Object.keys(pkg.peerDependencies || {})
     ],
-    plugins: [babel()]
+    plugins: [
+      nodeResolve({
+        extensions: ['.ts']
+      }),
+      typescript({ tsconfigOverride: noDeclarationFiles }),
+      babel()
+    ]
   },
 
   // ES for Browsers
   {
-    input: 'src/index.js',
+    input: 'src/index.ts',
     output: { file: 'es/redux.mjs', format: 'es', indent: false },
     plugins: [
-      nodeResolve(),
+      nodeResolve({
+        extensions: ['.ts']
+      }),
       replace({
         'process.env.NODE_ENV': JSON.stringify('production')
       }),
+      typescript({ tsconfigOverride: noDeclarationFiles }),
+      babel({
+        exclude: 'node_modules/**'
+      }),
       terser({
         compress: {
           pure_getters: true,
@@ -50,7 +71,7 @@ export default [
 
   // UMD Development
   {
-    input: 'src/index.js',
+    input: 'src/index.ts',
     output: {
       file: 'dist/redux.js',
       format: 'umd',
@@ -58,7 +79,10 @@ export default [
       indent: false
     },
     plugins: [
-      nodeResolve(),
+      nodeResolve({
+        extensions: ['.ts']
+      }),
+      typescript({ tsconfigOverride: noDeclarationFiles }),
       babel({
         exclude: 'node_modules/**'
       }),
@@ -70,7 +94,7 @@ export default [
 
   // UMD Production
   {
-    input: 'src/index.js',
+    input: 'src/index.ts',
     output: {
       file: 'dist/redux.min.js',
       format: 'umd',
@@ -78,7 +102,10 @@ export default [
       indent: false
     },
     plugins: [
-      nodeResolve(),
+      nodeResolve({
+        extensions: ['.ts']
+      }),
+      typescript({ tsconfigOverride: noDeclarationFiles }),
       babel({
         exclude: 'node_modules/**'
       }),
diff --git a/src/applyMiddleware.js b/src/applyMiddleware.js
deleted file mode 100644
index 3bfc647b45..0000000000
--- a/src/applyMiddleware.js
+++ /dev/null
@@ -1,41 +0,0 @@
-import compose from './compose'
-
-/**
- * Creates a store enhancer that applies middleware to the dispatch method
- * of the Redux store. This is handy for a variety of tasks, such as expressing
- * asynchronous actions in a concise manner, or logging every action payload.
- *
- * See `redux-thunk` package as an example of the Redux middleware.
- *
- * Because middleware is potentially asynchronous, this should be the first
- * store enhancer in the composition chain.
- *
- * Note that each middleware will be given the `dispatch` and `getState` functions
- * as named arguments.
- *
- * @param {...Function} middlewares The middleware chain to be applied.
- * @returns {Function} A store enhancer applying the middleware.
- */
-export default function applyMiddleware(...middlewares) {
-  return createStore => (...args) => {
-    const store = createStore(...args)
-    let dispatch = () => {
-      throw new Error(
-        'Dispatching while constructing your middleware is not allowed. ' +
-          'Other middleware would not be applied to this dispatch.'
-      )
-    }
-
-    const middlewareAPI = {
-      getState: store.getState,
-      dispatch: (...args) => dispatch(...args)
-    }
-    const chain = middlewares.map(middleware => middleware(middlewareAPI))
-    dispatch = compose(...chain)(store.dispatch)
-
-    return {
-      ...store,
-      dispatch
-    }
-  }
-}
diff --git a/src/applyMiddleware.ts b/src/applyMiddleware.ts
new file mode 100644
index 0000000000..f735843252
--- /dev/null
+++ b/src/applyMiddleware.ts
@@ -0,0 +1,82 @@
+import compose from './compose'
+import { Middleware, MiddlewareAPI } from './types/middleware'
+import { AnyAction } from './types/actions'
+import { StoreEnhancer, StoreCreator, Dispatch } from './types/store'
+import { Reducer } from './types/reducers'
+
+/**
+ * Creates a store enhancer that applies middleware to the dispatch method
+ * of the Redux store. This is handy for a variety of tasks, such as expressing
+ * asynchronous actions in a concise manner, or logging every action payload.
+ *
+ * See `redux-thunk` package as an example of the Redux middleware.
+ *
+ * Because middleware is potentially asynchronous, this should be the first
+ * store enhancer in the composition chain.
+ *
+ * Note that each middleware will be given the `dispatch` and `getState` functions
+ * as named arguments.
+ *
+ * @param middlewares The middleware chain to be applied.
+ * @returns A store enhancer applying the middleware.
+ *
+ * @template Ext Dispatch signature added by a middleware.
+ * @template S The type of the state supported by a middleware.
+ */
+export default function applyMiddleware(): StoreEnhancer
+export default function applyMiddleware<Ext1, S>(
+  middleware1: Middleware<Ext1, S, any>
+): StoreEnhancer<{ dispatch: Ext1 }>
+export default function applyMiddleware<Ext1, Ext2, S>(
+  middleware1: Middleware<Ext1, S, any>,
+  middleware2: Middleware<Ext2, S, any>
+): StoreEnhancer<{ dispatch: Ext1 & Ext2 }>
+export default function applyMiddleware<Ext1, Ext2, Ext3, S>(
+  middleware1: Middleware<Ext1, S, any>,
+  middleware2: Middleware<Ext2, S, any>,
+  middleware3: Middleware<Ext3, S, any>
+): StoreEnhancer<{ dispatch: Ext1 & Ext2 & Ext3 }>
+export default function applyMiddleware<Ext1, Ext2, Ext3, Ext4, S>(
+  middleware1: Middleware<Ext1, S, any>,
+  middleware2: Middleware<Ext2, S, any>,
+  middleware3: Middleware<Ext3, S, any>,
+  middleware4: Middleware<Ext4, S, any>
+): StoreEnhancer<{ dispatch: Ext1 & Ext2 & Ext3 & Ext4 }>
+export default function applyMiddleware<Ext1, Ext2, Ext3, Ext4, Ext5, S>(
+  middleware1: Middleware<Ext1, S, any>,
+  middleware2: Middleware<Ext2, S, any>,
+  middleware3: Middleware<Ext3, S, any>,
+  middleware4: Middleware<Ext4, S, any>,
+  middleware5: Middleware<Ext5, S, any>
+): StoreEnhancer<{ dispatch: Ext1 & Ext2 & Ext3 & Ext4 & Ext5 }>
+export default function applyMiddleware<Ext, S = any>(
+  ...middlewares: Middleware<any, S, any>[]
+): StoreEnhancer<{ dispatch: Ext }>
+export default function applyMiddleware(
+  ...middlewares: Middleware[]
+): StoreEnhancer<any> {
+  return (createStore: StoreCreator) => <S, A extends AnyAction>(
+    reducer: Reducer<S, A>,
+    ...args: any[]
+  ) => {
+    const store = createStore(reducer, ...args)
+    let dispatch: Dispatch = () => {
+      throw new Error(
+        'Dispatching while constructing your middleware is not allowed. ' +
+          'Other middleware would not be applied to this dispatch.'
+      )
+    }
+
+    const middlewareAPI: MiddlewareAPI = {
+      getState: store.getState,
+      dispatch: (action, ...args) => dispatch(action, ...args)
+    }
+    const chain = middlewares.map(middleware => middleware(middlewareAPI))
+    dispatch = compose<typeof dispatch>(...chain)(store.dispatch)
+
+    return {
+      ...store,
+      dispatch
+    }
+  }
+}
diff --git a/src/bindActionCreators.js b/src/bindActionCreators.ts
similarity index 52%
rename from src/bindActionCreators.js
rename to src/bindActionCreators.ts
index 0e47477def..073a3b58df 100644
--- a/src/bindActionCreators.js
+++ b/src/bindActionCreators.ts
@@ -1,6 +1,16 @@
-function bindActionCreator(actionCreator, dispatch) {
-  return function() {
-    return dispatch(actionCreator.apply(this, arguments))
+import { Dispatch } from './types/store'
+import {
+  AnyAction,
+  ActionCreator,
+  ActionCreatorsMapObject
+} from './types/actions'
+
+function bindActionCreator<A extends AnyAction = AnyAction>(
+  actionCreator: ActionCreator<A>,
+  dispatch: Dispatch
+) {
+  return function(this: any, ...args: any[]) {
+    return dispatch(actionCreator.apply(this, args))
   }
 }
 
@@ -13,19 +23,41 @@ function bindActionCreator(actionCreator, dispatch) {
  * For convenience, you can also pass an action creator as the first argument,
  * and get a dispatch wrapped function in return.
  *
- * @param {Function|Object} actionCreators An object whose values are action
+ * @param actionCreators An object whose values are action
  * creator functions. One handy way to obtain it is to use ES6 `import * as`
  * syntax. You may also pass a single function.
  *
- * @param {Function} dispatch The `dispatch` function available on your Redux
+ * @param dispatch The `dispatch` function available on your Redux
  * store.
  *
- * @returns {Function|Object} The object mimicking the original object, but with
+ * @returns The object mimicking the original object, but with
  * every action creator wrapped into the `dispatch` call. If you passed a
  * function as `actionCreators`, the return value will also be a single
  * function.
  */
-export default function bindActionCreators(actionCreators, dispatch) {
+export default function bindActionCreators<A, C extends ActionCreator<A>>(
+  actionCreator: C,
+  dispatch: Dispatch
+): C
+
+export default function bindActionCreators<
+  A extends ActionCreator<any>,
+  B extends ActionCreator<any>
+>(actionCreator: A, dispatch: Dispatch): B
+
+export default function bindActionCreators<
+  A,
+  M extends ActionCreatorsMapObject<A>
+>(actionCreators: M, dispatch: Dispatch): M
+export default function bindActionCreators<
+  M extends ActionCreatorsMapObject<any>,
+  N extends ActionCreatorsMapObject<any>
+>(actionCreators: M, dispatch: Dispatch): N
+
+export default function bindActionCreators(
+  actionCreators: ActionCreator<any> | ActionCreatorsMapObject,
+  dispatch: Dispatch
+) {
   if (typeof actionCreators === 'function') {
     return bindActionCreator(actionCreators, dispatch)
   }
@@ -39,7 +71,7 @@ export default function bindActionCreators(actionCreators, dispatch) {
     )
   }
 
-  const boundActionCreators = {}
+  const boundActionCreators: ActionCreatorsMapObject = {}
   for (const key in actionCreators) {
     const actionCreator = actionCreators[key]
     if (typeof actionCreator === 'function') {
diff --git a/src/combineReducers.js b/src/combineReducers.ts
similarity index 71%
rename from src/combineReducers.js
rename to src/combineReducers.ts
index bdbcfb2291..07f9f7bd22 100644
--- a/src/combineReducers.js
+++ b/src/combineReducers.ts
@@ -1,8 +1,16 @@
+import { Reducer } from './types/reducers'
+import { AnyAction, Action } from './types/actions'
 import ActionTypes from './utils/actionTypes'
 import warning from './utils/warning'
 import isPlainObject from './utils/isPlainObject'
-
-function getUndefinedStateErrorMessage(key, action) {
+import {
+  ReducersMapObject,
+  StateFromReducersMapObject,
+  ActionFromReducersMapObject
+} from './types/reducers'
+import { CombinedState } from './types/store'
+
+function getUndefinedStateErrorMessage(key: string, action: Action) {
   const actionType = action && action.type
   const actionDescription =
     (actionType && `action "${String(actionType)}"`) || 'an action'
@@ -15,10 +23,10 @@ function getUndefinedStateErrorMessage(key, action) {
 }
 
 function getUnexpectedStateShapeWarningMessage(
-  inputState,
-  reducers,
-  action,
-  unexpectedKeyCache
+  inputState: object,
+  reducers: ReducersMapObject,
+  action: Action,
+  unexpectedKeyCache: { [key: string]: true }
 ) {
   const reducerKeys = Object.keys(reducers)
   const argumentName =
@@ -34,9 +42,13 @@ function getUnexpectedStateShapeWarningMessage(
   }
 
   if (!isPlainObject(inputState)) {
+    const match = Object.prototype.toString
+      .call(inputState)
+      .match(/\s([a-z|A-Z]+)/)
+    const matchType = match ? match[1] : ''
     return (
       `The ${argumentName} has unexpected type of "` +
-      {}.toString.call(inputState).match(/\s([a-z|A-Z]+)/)[1] +
+      matchType +
       `". Expected argument to be an object with the following ` +
       `keys: "${reducerKeys.join('", "')}"`
     )
@@ -62,7 +74,7 @@ function getUnexpectedStateShapeWarningMessage(
   }
 }
 
-function assertReducerShape(reducers) {
+function assertReducerShape(reducers: ReducersMapObject) {
   Object.keys(reducers).forEach(key => {
     const reducer = reducers[key]
     const initialState = reducer(undefined, { type: ActionTypes.INIT })
@@ -100,19 +112,33 @@ function assertReducerShape(reducers) {
  * into a single state object, whose keys correspond to the keys of the passed
  * reducer functions.
  *
- * @param {Object} reducers An object whose values correspond to different
- * reducer functions that need to be combined into one. One handy way to obtain
- * it is to use ES6 `import * as reducers` syntax. The reducers may never return
- * undefined for any action. Instead, they should return their initial state
- * if the state passed to them was undefined, and the current state for any
- * unrecognized action.
+ * @template S Combined state object type.
+ *
+ * @param reducers An object whose values correspond to different reducer
+ *   functions that need to be combined into one. One handy way to obtain it
+ *   is to use ES6 `import * as reducers` syntax. The reducers may never
+ *   return undefined for any action. Instead, they should return their
+ *   initial state if the state passed to them was undefined, and the current
+ *   state for any unrecognized action.
  *
- * @returns {Function} A reducer function that invokes every reducer inside the
- * passed object, and builds a state object with the same shape.
+ * @returns A reducer function that invokes every reducer inside the passed
+ *   object, and builds a state object with the same shape.
  */
-export default function combineReducers(reducers) {
+export default function combineReducers<S>(
+  reducers: ReducersMapObject<S, any>
+): Reducer<CombinedState<S>>
+export default function combineReducers<S, A extends Action = AnyAction>(
+  reducers: ReducersMapObject<S, A>
+): Reducer<CombinedState<S>, A>
+export default function combineReducers<M extends ReducersMapObject<any, any>>(
+  reducers: M
+): Reducer<
+  CombinedState<StateFromReducersMapObject<M>>,
+  ActionFromReducersMapObject<M>
+>
+export default function combineReducers(reducers: ReducersMapObject) {
   const reducerKeys = Object.keys(reducers)
-  const finalReducers = {}
+  const finalReducers: ReducersMapObject = {}
   for (let i = 0; i < reducerKeys.length; i++) {
     const key = reducerKeys[i]
 
@@ -130,19 +156,22 @@ export default function combineReducers(reducers) {
 
   // This is used to make sure we don't warn about the same
   // keys multiple times.
-  let unexpectedKeyCache
+  let unexpectedKeyCache: { [key: string]: true }
   if (process.env.NODE_ENV !== 'production') {
     unexpectedKeyCache = {}
   }
 
-  let shapeAssertionError
+  let shapeAssertionError: Error
   try {
     assertReducerShape(finalReducers)
   } catch (e) {
     shapeAssertionError = e
   }
 
-  return function combination(state = {}, action) {
+  return function combination(
+    state: StateFromReducersMapObject<typeof reducers> = {},
+    action: AnyAction
+  ) {
     if (shapeAssertionError) {
       throw shapeAssertionError
     }
@@ -160,7 +189,7 @@ export default function combineReducers(reducers) {
     }
 
     let hasChanged = false
-    const nextState = {}
+    const nextState: StateFromReducersMapObject<typeof reducers> = {}
     for (let i = 0; i < finalReducerKeys.length; i++) {
       const key = finalReducerKeys[i]
       const reducer = finalReducers[key]
diff --git a/src/compose.js b/src/compose.js
deleted file mode 100644
index be6cc34ef1..0000000000
--- a/src/compose.js
+++ /dev/null
@@ -1,22 +0,0 @@
-/**
- * Composes single-argument functions from right to left. The rightmost
- * function can take multiple arguments as it provides the signature for
- * the resulting composite function.
- *
- * @param {...Function} funcs The functions to compose.
- * @returns {Function} A function obtained by composing the argument functions
- * from right to left. For example, compose(f, g, h) is identical to doing
- * (...args) => f(g(h(...args))).
- */
-
-export default function compose(...funcs) {
-  if (funcs.length === 0) {
-    return arg => arg
-  }
-
-  if (funcs.length === 1) {
-    return funcs[0]
-  }
-
-  return funcs.reduce((a, b) => (...args) => a(b(...args)))
-}
diff --git a/src/compose.ts b/src/compose.ts
new file mode 100644
index 0000000000..fcbfd70d7b
--- /dev/null
+++ b/src/compose.ts
@@ -0,0 +1,102 @@
+type Func0<R> = () => R
+type Func1<T1, R> = (a1: T1) => R
+type Func2<T1, T2, R> = (a1: T1, a2: T2) => R
+type Func3<T1, T2, T3, R> = (a1: T1, a2: T2, a3: T3, ...args: any[]) => R
+
+/**
+ * Composes single-argument functions from right to left. The rightmost
+ * function can take multiple arguments as it provides the signature for the
+ * resulting composite function.
+ *
+ * @param funcs The functions to compose.
+ * @returns A function obtained by composing the argument functions from right
+ *   to left. For example, `compose(f, g, h)` is identical to doing
+ *   `(...args) => f(g(h(...args)))`.
+ */
+export default function compose(): <R>(a: R) => R
+
+export default function compose<F extends Function>(f: F): F
+
+/* two functions */
+export default function compose<A, R>(f1: (b: A) => R, f2: Func0<A>): Func0<R>
+export default function compose<A, T1, R>(
+  f1: (b: A) => R,
+  f2: Func1<T1, A>
+): Func1<T1, R>
+export default function compose<A, T1, T2, R>(
+  f1: (b: A) => R,
+  f2: Func2<T1, T2, A>
+): Func2<T1, T2, R>
+export default function compose<A, T1, T2, T3, R>(
+  f1: (b: A) => R,
+  f2: Func3<T1, T2, T3, A>
+): Func3<T1, T2, T3, R>
+
+/* three functions */
+export default function compose<A, B, R>(
+  f1: (b: B) => R,
+  f2: (a: A) => B,
+  f3: Func0<A>
+): Func0<R>
+export default function compose<A, B, T1, R>(
+  f1: (b: B) => R,
+  f2: (a: A) => B,
+  f3: Func1<T1, A>
+): Func1<T1, R>
+export default function compose<A, B, T1, T2, R>(
+  f1: (b: B) => R,
+  f2: (a: A) => B,
+  f3: Func2<T1, T2, A>
+): Func2<T1, T2, R>
+export default function compose<A, B, T1, T2, T3, R>(
+  f1: (b: B) => R,
+  f2: (a: A) => B,
+  f3: Func3<T1, T2, T3, A>
+): Func3<T1, T2, T3, R>
+
+/* four functions */
+export default function compose<A, B, C, R>(
+  f1: (b: C) => R,
+  f2: (a: B) => C,
+  f3: (a: A) => B,
+  f4: Func0<A>
+): Func0<R>
+export default function compose<A, B, C, T1, R>(
+  f1: (b: C) => R,
+  f2: (a: B) => C,
+  f3: (a: A) => B,
+  f4: Func1<T1, A>
+): Func1<T1, R>
+export default function compose<A, B, C, T1, T2, R>(
+  f1: (b: C) => R,
+  f2: (a: B) => C,
+  f3: (a: A) => B,
+  f4: Func2<T1, T2, A>
+): Func2<T1, T2, R>
+export default function compose<A, B, C, T1, T2, T3, R>(
+  f1: (b: C) => R,
+  f2: (a: B) => C,
+  f3: (a: A) => B,
+  f4: Func3<T1, T2, T3, A>
+): Func3<T1, T2, T3, R>
+
+/* rest */
+export default function compose<R>(
+  f1: (b: any) => R,
+  ...funcs: Function[]
+): (...args: any[]) => R
+
+export default function compose<R>(...funcs: Function[]): (...args: any[]) => R
+
+export default function compose(...funcs: Function[]) {
+  if (funcs.length === 0) {
+    // infer the argument type so it is usable in inference down the line
+    return <T>(arg: T) => arg
+  }
+
+  if (funcs.length === 1) {
+    return funcs[0]
+  }
+
+  return funcs.reduce((a, b) => (...args: any) => a(b(...args)))
+}
diff --git a/src/createStore.js b/src/createStore.ts
similarity index 72%
rename from src/createStore.js
rename to src/createStore.ts
index cef9a2ecae..a75fc2a304 100644
--- a/src/createStore.js
+++ b/src/createStore.ts
@@ -1,5 +1,15 @@
 import $$observable from 'symbol-observable'
 
+import {
+  Store,
+  PreloadedState,
+  StoreEnhancer,
+  Dispatch,
+  Observer,
+  ExtendState
+} from './types/store'
+import { Action } from './types/actions'
+import { Reducer } from './types/reducers'
 import ActionTypes from './utils/actionTypes'
 import isPlainObject from './utils/isPlainObject'
 
@@ -11,24 +21,52 @@ import isPlainObject from './utils/isPlainObject'
  * parts of the state tree respond to actions, you may combine several reducers
  * into a single reducer function by using `combineReducers`.
  *
- * @param {Function} reducer A function that returns the next state tree, given
+ * @param reducer A function that returns the next state tree, given
  * the current state tree and the action to handle.
  *
- * @param {any} [preloadedState] The initial state. You may optionally specify it
+ * @param preloadedState The initial state. You may optionally specify it
  * to hydrate the state from the server in universal apps, or to restore a
  * previously serialized user session.
  * If you use `combineReducers` to produce the root reducer function, this must be
  * an object with the same shape as `combineReducers` keys.
  *
- * @param {Function} [enhancer] The store enhancer. You may optionally specify it
+ * @param enhancer The store enhancer. You may optionally specify it
  * to enhance the store with third-party capabilities such as middleware,
  * time travel, persistence, etc. The only store enhancer that ships with Redux
  * is `applyMiddleware()`.
  *
- * @returns {Store} A Redux store that lets you read the state, dispatch actions
+ * @returns A Redux store that lets you read the state, dispatch actions
  * and subscribe to changes.
  */
-export default function createStore(reducer, preloadedState, enhancer) {
+export default function createStore<
+  S,
+  A extends Action,
+  Ext = {},
+  StateExt = never
+>(
+  reducer: Reducer<S, A>,
+  enhancer?: StoreEnhancer<Ext, StateExt>
+): Store<ExtendState<S, StateExt>, A, StateExt, Ext> & Ext
+export default function createStore<
+  S,
+  A extends Action,
+  Ext = {},
+  StateExt = never
+>(
+  reducer: Reducer<S, A>,
+  preloadedState?: PreloadedState<S>,
+  enhancer?: StoreEnhancer<Ext, StateExt>
+): Store<ExtendState<S, StateExt>, A, StateExt, Ext> & Ext
+export default function createStore<
+  S,
+  A extends Action,
+  Ext = {},
+  StateExt = never
+>(
+  reducer: Reducer<S, A>,
+  preloadedState?: PreloadedState<S> | StoreEnhancer<Ext, StateExt>,
+  enhancer?: StoreEnhancer<Ext, StateExt>
+): Store<ExtendState<S, StateExt>, A, StateExt, Ext> & Ext {
   if (
     (typeof preloadedState === 'function' && typeof enhancer === 'function') ||
     (typeof enhancer === 'function' && typeof arguments[3] === 'function')
@@ -41,7 +79,7 @@ export default function createStore(reducer, preloadedState, enhancer) {
   }
 
   if (typeof preloadedState === 'function' && typeof enhancer === 'undefined') {
-    enhancer = preloadedState
+    enhancer = preloadedState as StoreEnhancer<Ext, StateExt>
     preloadedState = undefined
   }
 
@@ -50,7 +88,9 @@ export default function createStore(reducer, preloadedState, enhancer) {
       throw new Error('Expected the enhancer to be a function.')
     }
 
-    return enhancer(createStore)(reducer, preloadedState)
+    return enhancer(createStore)(reducer, preloadedState as PreloadedState<
+      S
+    >) as Store<ExtendState<S, StateExt>, A, StateExt, Ext> & Ext
   }
 
   if (typeof reducer !== 'function') {
@@ -58,8 +98,8 @@ export default function createStore(reducer, preloadedState, enhancer) {
   }
 
   let currentReducer = reducer
-  let currentState = preloadedState
-  let currentListeners = []
+  let currentState = preloadedState as S
+  let currentListeners: (() => void)[] | null = []
   let nextListeners = currentListeners
   let isDispatching = false
 
@@ -79,9 +119,9 @@ export default function createStore(reducer, preloadedState, enhancer) {
   /**
    * Reads the state tree managed by the store.
    *
-   * @returns {any} The current state tree of your application.
+   * @returns The current state tree of your application.
    */
-  function getState() {
+  function getState(): S {
     if (isDispatching) {
       throw new Error(
         'You may not call store.getState() while the reducer is executing. ' +
@@ -90,7 +130,7 @@ export default function createStore(reducer, preloadedState, enhancer) {
       )
     }
 
-    return currentState
+    return currentState as S
   }
 
   /**
@@ -113,10 +153,10 @@ export default function createStore(reducer, preloadedState, enhancer) {
    * registered before the `dispatch()` started will be called with the latest
    * state by the time it exits.
    *
-   * @param {Function} listener A callback to be invoked on every dispatch.
-   * @returns {Function} A function to remove this change listener.
+   * @param listener A callback to be invoked on every dispatch.
+   * @returns A function to remove this change listener.
    */
-  function subscribe(listener) {
+  function subscribe(listener: () => void) {
     if (typeof listener !== 'function') {
       throw new Error('Expected the listener to be a function.')
     }
@@ -170,18 +210,18 @@ export default function createStore(reducer, preloadedState, enhancer) {
    * example, see the documentation for the `redux-thunk` package. Even the
    * middleware will eventually dispatch plain object actions using this method.
    *
-   * @param {Object} action A plain object representing “what changed”. It is
+   * @param action A plain object representing “what changed”. It is
    * a good idea to keep actions serializable so you can record and replay user
    * sessions, or use the time travelling `redux-devtools`. An action must have
    * a `type` property which may not be `undefined`. It is a good idea to use
    * string constants for action types.
    *
-   * @returns {Object} For convenience, the same action object you dispatched.
+   * @returns For convenience, the same action object you dispatched.
    *
    * Note that, if you use a custom middleware, it may wrap `dispatch()` to
    * return something else (for example, a Promise you can await).
    */
-  function dispatch(action) {
+  function dispatch(action: A) {
     if (!isPlainObject(action)) {
       throw new Error(
         'Actions must be plain objects. ' +
@@ -223,26 +263,40 @@ export default function createStore(reducer, preloadedState, enhancer) {
    * load some of the reducers dynamically. You might also need this if you
    * implement a hot reloading mechanism for Redux.
    *
-   * @param {Function} nextReducer The reducer for the store to use instead.
-   * @returns {void}
+   * @param nextReducer The reducer for the store to use instead.
+   * @returns The same store instance with a new reducer in place.
    */
-  function replaceReducer(nextReducer) {
+  function replaceReducer<NewState, NewActions extends A>(
+    nextReducer: Reducer<NewState, NewActions>
+  ): Store<ExtendState<NewState, StateExt>, NewActions, StateExt, Ext> & Ext {
     if (typeof nextReducer !== 'function') {
       throw new Error('Expected the nextReducer to be a function.')
     }
 
-    currentReducer = nextReducer
+    // TODO: do this more elegantly
+    ;((currentReducer as unknown) as Reducer<
+      NewState,
+      NewActions
+    >) = nextReducer
 
     // This action has a similiar effect to ActionTypes.INIT.
     // Any reducers that existed in both the new and old rootReducer
     // will receive the previous state. This effectively populates
     // the new state tree with any relevant data from the old one.
-    dispatch({ type: ActionTypes.REPLACE })
+    dispatch({ type: ActionTypes.REPLACE } as A)
+    // change the type of the store by casting it to the new store
+    return (store as unknown) as Store<
+      ExtendState<NewState, StateExt>,
+      NewActions,
+      StateExt,
+      Ext
+    > &
+      Ext
   }
 
   /**
    * Interoperability point for observable/reactive libraries.
-   * @returns {observable} A minimal observable of state changes.
+   * @returns A minimal observable of state changes.
    * For more information, see the observable proposal:
    * https://github.com/tc39/proposal-observable
    */
@@ -251,20 +305,21 @@ export default function createStore(reducer, preloadedState, enhancer) {
     return {
       /**
        * The minimal observable subscription method.
-       * @param {Object} observer Any object that can be used as an observer.
+       * @param observer Any object that can be used as an observer.
        * The observer object should have a `next` method.
-       * @returns {subscription} An object with an `unsubscribe` method that can
+       * @returns An object with an `unsubscribe` method that can
        * be used to unsubscribe the observable from the store, and prevent further
        * emission of values from the observable.
        */
-      subscribe(observer) {
+      subscribe(observer: unknown) {
         if (typeof observer !== 'object' || observer === null) {
           throw new TypeError('Expected the observer to be an object.')
         }
 
         function observeState() {
-          if (observer.next) {
-            observer.next(getState())
+          const observerAsObserver = observer as Observer<S>
+          if (observerAsObserver.next) {
+            observerAsObserver.next(getState())
           }
         }
 
@@ -282,13 +337,14 @@ export default function createStore(reducer, preloadedState, enhancer) {
   // When a store is created, an "INIT" action is dispatched so that every
   // reducer returns their initial state. This effectively populates
   // the initial state tree.
-  dispatch({ type: ActionTypes.INIT })
+  dispatch({ type: ActionTypes.INIT } as A)
 
-  return {
-    dispatch,
+  const store = ({
+    dispatch: dispatch as Dispatch<A>,
     subscribe,
     getState,
     replaceReducer,
     [$$observable]: observable
-  }
+  } as unknown) as Store<ExtendState<S, StateExt>, A, StateExt, Ext> & Ext
+  return store
 }
diff --git a/src/index.js b/src/index.ts
similarity index 64%
rename from src/index.js
rename to src/index.ts
index efedbc4fed..40df5f346d 100644
--- a/src/index.js
+++ b/src/index.ts
@@ -1,3 +1,4 @@
+// functions
 import createStore from './createStore'
 import combineReducers from './combineReducers'
 import bindActionCreators from './bindActionCreators'
@@ -6,6 +7,37 @@ import compose from './compose'
 import warning from './utils/warning'
 import __DO_NOT_USE__ActionTypes from './utils/actionTypes'
 
+// types
+// store
+export {
+  CombinedState,
+  PreloadedState,
+  Dispatch,
+  Unsubscribe,
+  Observable,
+  Observer,
+  Store,
+  StoreCreator,
+  StoreEnhancer,
+  StoreEnhancerStoreCreator,
+  ExtendState
+} from './types/store'
+// reducers
+export {
+  Reducer,
+  ReducerFromReducersMapObject,
+  ReducersMapObject,
+  StateFromReducersMapObject,
+  ActionFromReducer,
+  ActionFromReducersMapObject
+} from './types/reducers'
+// action creators
+export { ActionCreator, ActionCreatorsMapObject } from './types/actions'
+// middleware
+export { MiddlewareAPI, Middleware } from './types/middleware'
+// actions
+export { Action, AnyAction } from './types/actions'
+
 /*
  * This is a dummy function to check if the function name has been altered by minification.
  * If the function has been minified and NODE_ENV !== 'production', warn the user.
diff --git a/src/types/actions.ts b/src/types/actions.ts
new file mode 100644
index 0000000000..75215ed7af
--- /dev/null
+++ b/src/types/actions.ts
@@ -0,0 +1,61 @@
+/**
+ * An *action* is a plain object that represents an intention to change the
+ * state. Actions are the only way to get data into the store. Any data,
+ * whether from UI events, network callbacks, or other sources such as
+ * WebSockets needs to eventually be dispatched as actions.
+ *
+ * Actions must have a `type` field that indicates the type of action being
+ * performed. Types can be defined as constants and imported from another
+ * module. It's better to use strings for `type` than Symbols because strings
+ * are serializable.
+ *
+ * Other than `type`, the structure of an action object is really up to you.
+ * If you're interested, check out Flux Standard Action for recommendations on
+ * how actions should be constructed.
+ *
+ * @template T the type of the action's `type` tag.
+ */
+export interface Action<T = any> {
+  type: T
+}
+
+/**
+ * An Action type which accepts any other properties.
+ * This is mainly for the use of the `Reducer` type.
+ * This is not part of `Action` itself to prevent types that extend `Action` from
+ * having an index signature.
+ */
+export interface AnyAction extends Action {
+  // Allows any extra properties to be defined in an action.
+  [extraProps: string]: any
+}
+
+/* action creators */
+
+/**
+ * An *action creator* is, quite simply, a function that creates an action. Do
+ * not confuse the two terms—again, an action is a payload of information, and
+ * an action creator is a factory that creates an action.
+ *
+ * Calling an action creator only produces an action, but does not dispatch
+ * it. You need to call the store's `dispatch` function to actually cause the
+ * mutation. Sometimes we say *bound action creators* to mean functions that
+ * call an action creator and immediately dispatch its result to a specific
+ * store instance.
+ *
+ * If an action creator needs to read the current state, perform an API call,
+ * or cause a side effect, like a routing transition, it should return an
+ * async action instead of an action.
+ *
+ * @template A Returned action type.
+ */
+export interface ActionCreator<A> {
+  (...args: any[]): A
+}
+
+/**
+ * Object whose values are action creator functions.
+ */
+export interface ActionCreatorsMapObject<A = any> {
+  [key: string]: ActionCreator<A>
+}
diff --git a/src/types/middleware.ts b/src/types/middleware.ts
new file mode 100644
index 0000000000..d631d988da
--- /dev/null
+++ b/src/types/middleware.ts
@@ -0,0 +1,31 @@
+import { Dispatch } from './store'
+import { AnyAction } from './actions'
+
+export interface MiddlewareAPI<D extends Dispatch = Dispatch, S = any> {
+  dispatch: D
+  getState(): S
+}
+
+/**
+ * A middleware is a higher-order function that composes a dispatch function
+ * to return a new dispatch function. It often turns async actions into
+ * actions.
+ *
+ * Middleware is composable using function composition. It is useful for
+ * logging actions, performing side effects like routing, or turning an
+ * asynchronous API call into a series of synchronous actions.
+ *
+ * @template DispatchExt Extra Dispatch signature added by this middleware.
+ * @template S The type of the state supported by this middleware.
+ * @template D The type of Dispatch of the store where this middleware is
+ *   installed.
+ */
+export interface Middleware<
+  _DispatchExt = {}, // TODO: remove unused component (breaking change)
+  S = any,
+  D extends Dispatch = Dispatch
+> {
+  (api: MiddlewareAPI<D, S>): (
+    next: Dispatch<AnyAction>
+  ) => (action: any) => any
+}
diff --git a/src/types/reducers.ts b/src/types/reducers.ts
new file mode 100644
index 0000000000..d0c41a9d33
--- /dev/null
+++ b/src/types/reducers.ts
@@ -0,0 +1,85 @@
+import { Action, AnyAction } from './actions'
+
+/* reducers */
+
+/**
+ * A *reducer* (also called a *reducing function*) is a function that accepts
+ * an accumulation and a value and returns a new accumulation. They are used
+ * to reduce a collection of values down to a single value
+ *
+ * Reducers are not unique to Redux—they are a fundamental concept in
+ * functional programming.  Even most non-functional languages, like
+ * JavaScript, have a built-in API for reducing. In JavaScript, it's
+ * `Array.prototype.reduce()`.
+ *
+ * In Redux, the accumulated value is the state object, and the values being
+ * accumulated are actions. Reducers calculate a new state given the previous
+ * state and an action. They must be *pure functions*—functions that return
+ * the exact same output for given inputs. They should also be free of
+ * side-effects. This is what enables exciting features like hot reloading and
+ * time travel.
+ *
+ * Reducers are the most important concept in Redux.
+ *
+ * *Do not put API calls into reducers.*
+ *
+ * @template S The type of state consumed and produced by this reducer.
+ * @template A The type of actions the reducer can potentially respond to.
+ */
+export type Reducer<S = any, A extends Action = AnyAction> = (
+  state: S | undefined,
+  action: A
+) => S
+
+/**
+ * Object whose values correspond to different reducer functions.
+ *
+ * @template A The type of actions the reducers can potentially respond to.
+ */
+export type ReducersMapObject<S = any, A extends Action = Action> = {
+  [K in keyof S]: Reducer<S[K], A>
+}
+
+/**
+ * Infer a combined state shape from a `ReducersMapObject`.
+ *
+ * @template M Object map of reducers as provided to `combineReducers(map: M)`.
+ */
+export type StateFromReducersMapObject<M> = M extends ReducersMapObject<
+  any,
+  any
+>
+  ? { [P in keyof M]: M[P] extends Reducer<infer S, any> ? S : never }
+  : never
+
+/**
+ * Infer reducer union type from a `ReducersMapObject`.
+ *
+ * @template M Object map of reducers as provided to `combineReducers(map: M)`.
+ */
+export type ReducerFromReducersMapObject<M> = M extends {
+  [P in keyof M]: infer R
+}
+  ? R extends Reducer<any, any>
+    ? R
+    : never
+  : never
+
+/**
+ * Infer action type from a reducer function.
+ *
+ * @template R Type of reducer.
+ */
+export type ActionFromReducer<R> = R extends Reducer<any, infer A> ? A : never
+
+/**
+ * Infer action union type from a `ReducersMapObject`.
+ *
+ * @template M Object map of reducers as provided to `combineReducers(map: M)`.
+ */
+export type ActionFromReducersMapObject<M> = M extends ReducersMapObject<
+  any,
+  any
+>
+  ? ActionFromReducer<ReducerFromReducersMapObject<M>>
+  : never
diff --git a/src/types/store.ts b/src/types/store.ts
new file mode 100644
index 0000000000..aaebb0ad21
--- /dev/null
+++ b/src/types/store.ts
@@ -0,0 +1,269 @@
+/// <reference types="symbol-observable" />
+import { Action, AnyAction } from './actions'
+import { Reducer } from './reducers'
+
+/**
+ * Extend the state
+ *
+ * This is used by store enhancers and store creators to extend state.
+ * If there is no state extension, it just returns the state, as is, otherwise
+ * it returns the state joined with its extension.
+ *
+ * Reference for future devs:
+ * https://github.com/microsoft/TypeScript/issues/31751#issuecomment-498526919
+ */
+export type ExtendState<State, Extension> = [Extension] extends [never]
+  ? State
+  : State & Extension
+
+/**
+ * Internal "virtual" symbol used to make the `CombinedState` type unique.
+ */
+declare const $CombinedState: unique symbol
+
+/**
+ * State base type for reducers created with `combineReducers()`.
+ *
+ * This type allows the `createStore()` method to infer which levels of the
+ * preloaded state can be partial.
+ *
+ * Because Typescript is really duck-typed, a type needs to have some
+ * identifying property to differentiate it from other types with matching
+ * prototypes for type checking purposes. That's why this type has the
+ * `$CombinedState` symbol property. Without the property, this type would
+ * match any object. The symbol doesn't really exist because it's an internal
+ * (i.e. not exported), and internally we never check its value. Since it's a
+ * symbol property, it's not expected to be unumerable, and the value is
+ * typed as always undefined, so its never expected to have a meaningful
+ * value anyway. It just makes this type distinquishable from plain `{}`.
+ */
+export type CombinedState<S> = { readonly [$CombinedState]?: undefined } & S
+
+/**
+ * Recursively makes combined state objects partial. Only combined state _root
+ * objects_ (i.e. the generated higher level object with keys mapping to
+ * individual reducers) are partial.
+ */
+export type PreloadedState<S> = Required<S> extends {
+  [$CombinedState]: undefined
+}
+  ? S extends CombinedState<infer S1>
+    ? {
+        [K in keyof S1]?: S1[K] extends object ? PreloadedState<S1[K]> : S1[K]
+      }
+    : never
+  : {
+      [K in keyof S]: S[K] extends object ? PreloadedState<S[K]> : S[K]
+    }
+
+/**
+ * A *dispatching function* (or simply *dispatch function*) is a function that
+ * accepts an action or an async action; it then may or may not dispatch one
+ * or more actions to the store.
+ *
+ * We must distinguish between dispatching functions in general and the base
+ * `dispatch` function provided by the store instance without any middleware.
+ *
+ * The base dispatch function *always* synchronously sends an action to the
+ * store's reducer, along with the previous state returned by the store, to
+ * calculate a new state. It expects actions to be plain objects ready to be
+ * consumed by the reducer.
+ *
+ * Middleware wraps the base dispatch function. It allows the dispatch
+ * function to handle async actions in addition to actions. Middleware may
+ * transform, delay, ignore, or otherwise interpret actions or async actions
+ * before passing them to the next middleware.
+ *
+ * @template A The type of things (actions or otherwise) which may be
+ *   dispatched.
+ */
+export interface Dispatch<A extends Action = AnyAction> {
+  <T extends A>(action: T, ...extraArgs: any[]): T
+}
+
+/**
+ * Function to remove listener added by `Store.subscribe()`.
+ */
+export interface Unsubscribe {
+  (): void
+}
+
+/**
+ * A minimal observable of state changes.
+ * For more information, see the observable proposal:
+ * https://github.com/tc39/proposal-observable
+ */
+export type Observable<T> = {
+  /**
+   * The minimal observable subscription method.
+   * @param {Object} observer Any object that can be used as an observer.
+   * The observer object should have a `next` method.
+   * @returns {subscription} An object with an `unsubscribe` method that can
+   * be used to unsubscribe the observable from the store, and prevent further
+   * emission of values from the observable.
+   */
+  subscribe: (observer: Observer<T>) => { unsubscribe: Unsubscribe }
+  [Symbol.observable](): Observable<T>
+}
+
+/**
+ * An Observer is used to receive data from an Observable, and is supplied as
+ * an argument to subscribe.
+ */
+export type Observer<T> = {
+  next?(value: T): void
+}
+
+/**
+ * A store is an object that holds the application's state tree.
+ * There should only be a single store in a Redux app, as the composition
+ * happens on the reducer level.
+ *
+ * @template S The type of state held by this store.
+ * @template A the type of actions which may be dispatched by this store.
+ * @template StateExt any extension to state from store enhancers
+ * @template Ext any extensions to the store from store enhancers
+ */
+export interface Store<
+  S = any,
+  A extends Action = AnyAction,
+  StateExt = never,
+  Ext = {}
+> {
+  /**
+   * Dispatches an action. It is the only way to trigger a state change.
+   *
+   * The `reducer` function, used to create the store, will be called with the
+   * current state tree and the given `action`. Its return value will be
+   * considered the **next** state of the tree, and the change listeners will
+   * be notified.
+   *
+   * The base implementation only supports plain object actions. If you want
+   * to dispatch a Promise, an Observable, a thunk, or something else, you
+   * need to wrap your store creating function into the corresponding
+   * middleware. For example, see the documentation for the `redux-thunk`
+   * package. Even the middleware will eventually dispatch plain object
+   * actions using this method.
+   *
+   * @param action A plain object representing “what changed”. It is a good
+   *   idea to keep actions serializable so you can record and replay user
+   *   sessions, or use the time travelling `redux-devtools`. An action must
+   *   have a `type` property which may not be `undefined`. It is a good idea
+   *   to use string constants for action types.
+   *
+   * @returns For convenience, the same action object you dispatched.
+   *
+   * Note that, if you use a custom middleware, it may wrap `dispatch()` to
+   * return something else (for example, a Promise you can await).
+   */
+  dispatch: Dispatch<A>
+
+  /**
+   * Reads the state tree managed by the store.
+   *
+   * @returns The current state tree of your application.
+   */
+  getState(): S
+
+  /**
+   * Adds a change listener. It will be called any time an action is
+   * dispatched, and some part of the state tree may potentially have changed.
+   * You may then call `getState()` to read the current state tree inside the
+   * callback.
+   *
+   * You may call `dispatch()` from a change listener, with the following
+   * caveats:
+   *
+   * 1. The subscriptions are snapshotted just before every `dispatch()` call.
+   * If you subscribe or unsubscribe while the listeners are being invoked,
+   * this will not have any effect on the `dispatch()` that is currently in
+   * progress. However, the next `dispatch()` call, whether nested or not,
+   * will use a more recent snapshot of the subscription list.
+   *
+   * 2. The listener should not expect to see all states changes, as the state
+   * might have been updated multiple times during a nested `dispatch()` before
+   * the listener is called. It is, however, guaranteed that all subscribers
+   * registered before the `dispatch()` started will be called with the latest
+   * state by the time it exits.
+   *
+   * @param listener A callback to be invoked on every dispatch.
+   * @returns A function to remove this change listener.
+   */
+  subscribe(listener: () => void): Unsubscribe
+
+  /**
+   * Replaces the reducer currently used by the store to calculate the state.
+   *
+   * You might need this if your app implements code splitting and you want to
+   * load some of the reducers dynamically. You might also need this if you
+   * implement a hot reloading mechanism for Redux.
+   *
+   * @param nextReducer The reducer for the store to use instead.
+   */
+  replaceReducer<NewState, NewActions extends Action>(
+    nextReducer: Reducer<NewState, NewActions>
+  ): Store<ExtendState<NewState, StateExt>, NewActions, StateExt, Ext> & Ext
+
+  /**
+   * Interoperability point for observable/reactive libraries.
+   * @returns {observable} A minimal observable of state changes.
+   * For more information, see the observable proposal:
+   * https://github.com/tc39/proposal-observable
+   */
+  [Symbol.observable](): Observable<S>
+}
+
+/**
+ * A store creator is a function that creates a Redux store. Like with
+ * dispatching function, we must distinguish the base store creator,
+ * `createStore(reducer, preloadedState)` exported from the Redux package, from
+ * store creators that are returned from the store enhancers.
+ *
+ * @template S The type of state to be held by the store.
+ * @template A The type of actions which may be dispatched.
+ * @template Ext Store extension that is mixed in to the Store type.
+ * @template StateExt State extension that is mixed into the state type.
+ */
+export interface StoreCreator {
+  <S, A extends Action, Ext = {}, StateExt = never>(
+    reducer: Reducer<S, A>,
+    enhancer?: StoreEnhancer<Ext, StateExt>
+  ): Store<ExtendState<S, StateExt>, A, StateExt, Ext> & Ext
+  <S, A extends Action, Ext = {}, StateExt = never>(
+    reducer: Reducer<S, A>,
+    preloadedState?: PreloadedState<S>,
+    enhancer?: StoreEnhancer<Ext>
+  ): Store<ExtendState<S, StateExt>, A, StateExt, Ext> & Ext
+}
+
+/**
+ * A store enhancer is a higher-order function that composes a store creator
+ * to return a new, enhanced store creator. This is similar to middleware in
+ * that it allows you to alter the store interface in a composable way.
+ *
+ * Store enhancers are much the same concept as higher-order components in
+ * React, which are also occasionally called “component enhancers”.
+ *
+ * Because a store is not an instance, but rather a plain-object collection of
+ * functions, copies can be easily created and modified without mutating the
+ * original store. There is an example in `compose` documentation
+ * demonstrating that.
+ *
+ * Most likely you'll never write a store enhancer, but you may use the one
+ * provided by the developer tools. It is what makes time travel possible
+ * without the app being aware it is happening. Amusingly, the Redux
+ * middleware implementation is itself a store enhancer.
+ *
+ * @template Ext Store extension that is mixed into the Store type.
+ * @template StateExt State extension that is mixed into the state type.
+ */
+export type StoreEnhancer<Ext = {}, StateExt = never> = (
+  next: StoreEnhancerStoreCreator<Ext, StateExt>
+) => StoreEnhancerStoreCreator<Ext, StateExt>
+export type StoreEnhancerStoreCreator<Ext = {}, StateExt = never> = <
+  S = any,
+  A extends Action = AnyAction
+>(
+  reducer: Reducer<S, A>,
+  preloadedState?: PreloadedState<S>
+) => Store<ExtendState<S, StateExt>, A, StateExt, Ext> & Ext
diff --git a/src/utils/actionTypes.js b/src/utils/actionTypes.ts
similarity index 100%
rename from src/utils/actionTypes.js
rename to src/utils/actionTypes.ts
diff --git a/src/utils/isPlainObject.js b/src/utils/isPlainObject.ts
similarity index 58%
rename from src/utils/isPlainObject.js
rename to src/utils/isPlainObject.ts
index 15ce724283..9d1956328a 100644
--- a/src/utils/isPlainObject.js
+++ b/src/utils/isPlainObject.ts
@@ -1,8 +1,8 @@
 /**
- * @param {any} obj The object to inspect.
- * @returns {boolean} True if the argument appears to be a plain object.
+ * @param obj The object to inspect.
+ * @returns True if the argument appears to be a plain object.
  */
-export default function isPlainObject(obj) {
+export default function isPlainObject(obj: any): boolean {
   if (typeof obj !== 'object' || obj === null) return false
 
   let proto = obj
diff --git a/src/utils/warning.js b/src/utils/warning.ts
similarity index 81%
rename from src/utils/warning.js
rename to src/utils/warning.ts
index d35e99b726..cefc5d1da6 100644
--- a/src/utils/warning.js
+++ b/src/utils/warning.ts
@@ -1,10 +1,9 @@
 /**
  * Prints a warning in the console if it exists.
  *
- * @param {String} message The warning message.
- * @returns {void}
+ * @param message The warning message.
  */
-export default function warning(message) {
+export default function warning(message: string): void {
   /* eslint-disable no-console */
   if (typeof console !== 'undefined' && typeof console.error === 'function') {
     console.error(message)
diff --git a/test/applyMiddleware.spec.js b/test/applyMiddleware.spec.ts
similarity index 77%
rename from test/applyMiddleware.spec.js
rename to test/applyMiddleware.spec.ts
index 2efdc9f83f..fefd849b15 100644
--- a/test/applyMiddleware.spec.js
+++ b/test/applyMiddleware.spec.ts
@@ -1,4 +1,4 @@
-import { createStore, applyMiddleware } from '../'
+import { createStore, applyMiddleware, Middleware, AnyAction, Action } from '..'
 import * as reducers from './helpers/reducers'
 import { addTodo, addTodoAsync, addTodoIfEmpty } from './helpers/actionCreators'
 import { thunk } from './helpers/middleware'
@@ -51,7 +51,11 @@ describe('applyMiddleware', () => {
     const spy = jest.fn()
     const store = applyMiddleware(test(spy), thunk)(createStore)(reducers.todos)
 
-    return store.dispatch(addTodoAsync('Use Redux')).then(() => {
+    // the typing for redux-thunk is super complex, so we will use an as unknown hack
+    const dispatchedValue = (store.dispatch(
+      addTodoAsync('Use Redux')
+    ) as unknown) as Promise<void>
+    return dispatchedValue.then(() => {
       expect(spy.mock.calls.length).toEqual(2)
     })
   })
@@ -87,7 +91,11 @@ describe('applyMiddleware', () => {
       }
     ])
 
-    store.dispatch(addTodoAsync('Maybe')).then(() => {
+    // the typing for redux-thunk is super complex, so we will use an "as unknown" hack
+    const dispatchedValue = (store.dispatch(
+      addTodoAsync('Maybe')
+    ) as unknown) as Promise<void>
+    dispatchedValue.then(() => {
       expect(store.getState()).toEqual([
         {
           id: 1,
@@ -110,8 +118,16 @@ describe('applyMiddleware', () => {
     const spy = jest.fn()
     const testCallArgs = ['test']
 
-    function multiArgMiddleware() {
-      return next => (action, callArgs) => {
+    interface MultiDispatch<A extends Action = AnyAction> {
+      <T extends A>(action: T, extraArg?: string[]): T
+    }
+
+    const multiArgMiddleware: Middleware<
+      MultiDispatch,
+      any,
+      MultiDispatch
+    > = _store => {
+      return next => (action, callArgs?: any) => {
         if (Array.isArray(callArgs)) {
           return action(...callArgs)
         }
@@ -120,7 +136,7 @@ describe('applyMiddleware', () => {
     }
 
     function dummyMiddleware({ dispatch }) {
-      return next => action => dispatch(action, testCallArgs)
+      return _next => action => dispatch(action, testCallArgs)
     }
 
     const store = createStore(
diff --git a/test/bindActionCreators.spec.js b/test/bindActionCreators.spec.ts
similarity index 88%
rename from test/bindActionCreators.spec.js
rename to test/bindActionCreators.spec.ts
index a9e2f2a6ea..5efecb6902 100644
--- a/test/bindActionCreators.spec.js
+++ b/test/bindActionCreators.spec.ts
@@ -1,4 +1,4 @@
-import { bindActionCreators, createStore } from '../'
+import { bindActionCreators, createStore, ActionCreator } from '..'
 import { todos } from './helpers/reducers'
 import * as actionCreators from './helpers/actionCreators'
 
@@ -46,15 +46,17 @@ describe('bindActionCreators', () => {
   })
 
   it('skips non-function values in the passed object', () => {
+    // as this is testing against invalid values, we will cast to unknown and then back to ActionCreator<any>
+    // in a typescript environment this test is unnecessary, but required in javascript
     const boundActionCreators = bindActionCreators(
-      {
+      ({
         ...actionCreators,
         foo: 42,
         bar: 'baz',
         wow: undefined,
         much: {},
         test: null
-      },
+      } as unknown) as ActionCreator<any>,
       store.dispatch
     )
     expect(Object.keys(boundActionCreators)).toEqual(
@@ -91,7 +93,10 @@ describe('bindActionCreators', () => {
 
   it('throws for a primitive actionCreator', () => {
     expect(() => {
-      bindActionCreators('string', store.dispatch)
+      bindActionCreators(
+        ('string' as unknown) as ActionCreator<any>,
+        store.dispatch
+      )
     }).toThrow(
       'bindActionCreators expected an object or a function, instead received string. ' +
         'Did you write "import ActionCreators from" instead of "import * as ActionCreators from"?'
diff --git a/test/combineReducers.spec.js b/test/combineReducers.spec.ts
similarity index 74%
rename from test/combineReducers.spec.js
rename to test/combineReducers.spec.ts
index e0e69aa110..161e00658b 100644
--- a/test/combineReducers.spec.js
+++ b/test/combineReducers.spec.ts
@@ -2,34 +2,40 @@
 import {
   createStore,
   combineReducers,
-  __DO_NOT_USE__ActionTypes as ActionTypes
-} from '../'
+  Reducer,
+  AnyAction,
+  __DO_NOT_USE__ActionTypes as ActionTypes,
+  CombinedState
+} from '..'
 
 describe('Utils', () => {
   describe('combineReducers', () => {
     it('returns a composite reducer that maps the state keys to given reducers', () => {
       const reducer = combineReducers({
-        counter: (state = 0, action) =>
+        counter: (state: number = 0, action) =>
           action.type === 'increment' ? state + 1 : state,
-        stack: (state = [], action) =>
+        stack: (state: any[] = [], action) =>
           action.type === 'push' ? [...state, action.value] : state
       })
 
-      const s1 = reducer({}, { type: 'increment' })
+      const s1 = reducer(undefined, { type: 'increment' })
       expect(s1).toEqual({ counter: 1, stack: [] })
       const s2 = reducer(s1, { type: 'push', value: 'a' })
       expect(s2).toEqual({ counter: 1, stack: ['a'] })
     })
 
     it('ignores all props which are not a function', () => {
+      // we double-cast because these conditions can only happen in a javascript setting
       const reducer = combineReducers({
-        fake: true,
-        broken: 'string',
-        another: { nested: 'object' },
+        fake: (true as unknown) as Reducer,
+        broken: ('string' as unknown) as Reducer,
+        another: ({ nested: 'object' } as unknown) as Reducer,
         stack: (state = []) => state
       })
 
-      expect(Object.keys(reducer({}, { type: 'push' }))).toEqual(['stack'])
+      expect(Object.keys(reducer(undefined, { type: 'push' }))).toEqual([
+        'stack'
+      ])
     })
 
     it('warns if a reducer prop is undefined', () => {
@@ -55,7 +61,7 @@ describe('Utils', () => {
 
     it('throws an error if a reducer returns undefined handling an action', () => {
       const reducer = combineReducers({
-        counter(state = 0, action) {
+        counter(state: number = 0, action) {
           switch (action && action.type) {
             case 'increment':
               return state + 1
@@ -77,12 +83,14 @@ describe('Utils', () => {
       expect(() => reducer({ counter: 0 }, null)).toThrow(
         /"counter".*an action/
       )
-      expect(() => reducer({ counter: 0 }, {})).toThrow(/"counter".*an action/)
+      expect(() =>
+        reducer({ counter: 0 }, ({} as unknown) as AnyAction)
+      ).toThrow(/"counter".*an action/)
     })
 
     it('throws an error on first call if a reducer returns undefined initializing', () => {
       const reducer = combineReducers({
-        counter(state, action) {
+        counter(state: number, action) {
           switch (action.type) {
             case 'increment':
               return state + 1
@@ -93,7 +101,9 @@ describe('Utils', () => {
           }
         }
       })
-      expect(() => reducer({})).toThrow(/"counter".*initialization/)
+      expect(() => reducer(undefined, { type: '' })).toThrow(
+        /"counter".*initialization/
+      )
     })
 
     it('catches error thrown in reducer when initializing and re-throw', () => {
@@ -102,14 +112,16 @@ describe('Utils', () => {
           throw new Error('Error thrown in reducer')
         }
       })
-      expect(() => reducer({})).toThrow(/Error thrown in reducer/)
+      expect(() =>
+        reducer(undefined, (undefined as unknown) as AnyAction)
+      ).toThrow(/Error thrown in reducer/)
     })
 
     it('allows a symbol to be used as an action type', () => {
       const increment = Symbol('INCREMENT')
 
       const reducer = combineReducers({
-        counter(state = 0, action) {
+        counter(state: number = 0, action) {
           switch (action.type) {
             case increment:
               return state + 1
@@ -135,7 +147,7 @@ describe('Utils', () => {
         }
       })
 
-      const initialState = reducer(undefined, '@@INIT')
+      const initialState = reducer(undefined, { type: '@@INIT' })
       expect(reducer(initialState, { type: 'FOO' })).toBe(initialState)
     })
 
@@ -144,7 +156,7 @@ describe('Utils', () => {
         child1(state = {}) {
           return state
         },
-        child2(state = { count: 0 }, action) {
+        child2(state: { count: number } = { count: 0 }, action) {
           switch (action.type) {
             case 'increment':
               return { count: state.count + 1 }
@@ -157,7 +169,7 @@ describe('Utils', () => {
         }
       })
 
-      const initialState = reducer(undefined, '@@INIT')
+      const initialState = reducer(undefined, { type: '@@INIT' })
       expect(reducer(initialState, { type: 'increment' })).not.toBe(
         initialState
       )
@@ -165,7 +177,7 @@ describe('Utils', () => {
 
     it('throws an error on first call if a reducer attempts to handle a private action', () => {
       const reducer = combineReducers({
-        counter(state, action) {
+        counter(state: number, action) {
           switch (action.type) {
             case 'increment':
               return state + 1
@@ -179,7 +191,9 @@ describe('Utils', () => {
           }
         }
       })
-      expect(() => reducer()).toThrow(/"counter".*private/)
+      expect(() =>
+        reducer(undefined, (undefined as unknown) as AnyAction)
+      ).toThrow(/"counter".*private/)
     })
 
     it('warns if no reducers are passed to combineReducers', () => {
@@ -188,7 +202,7 @@ describe('Utils', () => {
       console.error = spy
 
       const reducer = combineReducers({})
-      reducer({})
+      reducer(undefined, { type: '' })
       expect(spy.mock.calls[0][0]).toMatch(
         /Store does not have a valid reducer/
       )
@@ -200,9 +214,17 @@ describe('Utils', () => {
     it('warns if input state does not match reducer shape', () => {
       const preSpy = console.error
       const spy = jest.fn()
+      const nullAction = (undefined as unknown) as AnyAction
       console.error = spy
 
-      const reducer = combineReducers({
+      interface ShapeState {
+        foo: { bar: number }
+        baz: { qux: number }
+      }
+
+      type ShapeMismatchState = CombinedState<ShapeState>
+
+      const reducer = combineReducers<ShapeState>({
         foo(state = { bar: 1 }) {
           return state
         },
@@ -211,44 +233,51 @@ describe('Utils', () => {
         }
       })
 
-      reducer()
+      reducer(undefined, nullAction)
       expect(spy.mock.calls.length).toBe(0)
 
-      reducer({ foo: { bar: 2 } })
+      reducer(({ foo: { bar: 2 } } as unknown) as ShapeState, nullAction)
       expect(spy.mock.calls.length).toBe(0)
 
-      reducer({
-        foo: { bar: 2 },
-        baz: { qux: 4 }
-      })
+      reducer(
+        {
+          foo: { bar: 2 },
+          baz: { qux: 4 }
+        },
+        nullAction
+      )
       expect(spy.mock.calls.length).toBe(0)
 
-      createStore(reducer, { bar: 2 })
+      createStore(reducer, ({ bar: 2 } as unknown) as ShapeState)
       expect(spy.mock.calls[0][0]).toMatch(
         /Unexpected key "bar".*createStore.*instead: "foo", "baz"/
       )
 
-      createStore(reducer, { bar: 2, qux: 4, thud: 5 })
+      createStore(reducer, ({
+        bar: 2,
+        qux: 4,
+        thud: 5
+      } as unknown) as ShapeState)
       expect(spy.mock.calls[1][0]).toMatch(
         /Unexpected keys "qux", "thud".*createStore.*instead: "foo", "baz"/
       )
 
-      createStore(reducer, 1)
+      createStore(reducer, (1 as unknown) as ShapeState)
       expect(spy.mock.calls[2][0]).toMatch(
         /createStore has unexpected type of "Number".*keys: "foo", "baz"/
       )
 
-      reducer({ corge: 2 })
+      reducer(({ corge: 2 } as unknown) as ShapeState, nullAction)
       expect(spy.mock.calls[3][0]).toMatch(
         /Unexpected key "corge".*reducer.*instead: "foo", "baz"/
       )
 
-      reducer({ fred: 2, grault: 4 })
+      reducer(({ fred: 2, grault: 4 } as unknown) as ShapeState, nullAction)
       expect(spy.mock.calls[4][0]).toMatch(
         /Unexpected keys "fred", "grault".*reducer.*instead: "foo", "baz"/
       )
 
-      reducer(1)
+      reducer((1 as unknown) as ShapeState, nullAction)
       expect(spy.mock.calls[5][0]).toMatch(
         /reducer has unexpected type of "Number".*keys: "foo", "baz"/
       )
@@ -261,25 +290,32 @@ describe('Utils', () => {
       const preSpy = console.error
       const spy = jest.fn()
       console.error = spy
+      const nullAction = { type: '' }
 
       const foo = (state = { foo: 1 }) => state
       const bar = (state = { bar: 2 }) => state
 
       expect(spy.mock.calls.length).toBe(0)
 
+      interface WarnState {
+        foo: { foo: number }
+        bar: { bar: number }
+      }
+
       const reducer = combineReducers({ foo, bar })
-      const state = { foo: 1, bar: 2, qux: 3 }
+      const state = ({ foo: 1, bar: 2, qux: 3 } as unknown) as WarnState
+      const bazState = ({ ...state, baz: 5 } as unknown) as WarnState
 
-      reducer(state, {})
-      reducer(state, {})
-      reducer(state, {})
-      reducer(state, {})
+      reducer(state, nullAction)
+      reducer(state, nullAction)
+      reducer(state, nullAction)
+      reducer(state, nullAction)
       expect(spy.mock.calls.length).toBe(1)
 
-      reducer({ ...state, baz: 5 }, {})
-      reducer({ ...state, baz: 5 }, {})
-      reducer({ ...state, baz: 5 }, {})
-      reducer({ ...state, baz: 5 }, {})
+      reducer(bazState, nullAction)
+      reducer({ ...bazState }, nullAction)
+      reducer({ ...bazState }, nullAction)
+      reducer({ ...bazState }, nullAction)
       expect(spy.mock.calls.length).toBe(2)
 
       spy.mockClear()
diff --git a/test/compose.spec.js b/test/compose.spec.ts
similarity index 66%
rename from test/compose.spec.js
rename to test/compose.spec.ts
index 392b403f98..d25131cdeb 100644
--- a/test/compose.spec.js
+++ b/test/compose.spec.ts
@@ -1,10 +1,10 @@
-import { compose } from '../'
+import { compose } from '..'
 
 describe('Utils', () => {
   describe('compose', () => {
     it('composes from right to left', () => {
-      const double = x => x * 2
-      const square = x => x * x
+      const double = (x: number) => x * 2
+      const square = (x: number) => x * x
       expect(compose(square)(5)).toBe(25)
       expect(
         compose(
@@ -22,10 +22,10 @@ describe('Utils', () => {
     })
 
     it('composes functions from right to left', () => {
-      const a = next => x => next(x + 'a')
-      const b = next => x => next(x + 'b')
-      const c = next => x => next(x + 'c')
-      const final = x => x
+      const a = (next: (x: string) => string) => (x: string) => next(x + 'a')
+      const b = (next: (x: string) => string) => (x: string) => next(x + 'b')
+      const c = (next: (x: string) => string) => (x: string) => next(x + 'c')
+      const final = (x: string) => x
 
       expect(
         compose(
@@ -51,14 +51,15 @@ describe('Utils', () => {
     })
 
     it('throws at runtime if argument is not a function', () => {
-      const square = x => x * x
-      const add = (x, y) => x + y
+      type sFunc = (x: number, y: number) => number
+      const square = (x: number, _: number) => x * x
+      const add = (x: number, y: number) => x + y
 
       expect(() =>
         compose(
           square,
           add,
-          false
+          (false as unknown) as sFunc
         )(1, 2)
       ).toThrow()
       expect(() =>
@@ -72,28 +73,28 @@ describe('Utils', () => {
         compose(
           square,
           add,
-          true
+          (true as unknown) as sFunc
         )(1, 2)
       ).toThrow()
       expect(() =>
         compose(
           square,
           add,
-          NaN
+          (NaN as unknown) as sFunc
         )(1, 2)
       ).toThrow()
       expect(() =>
         compose(
           square,
           add,
-          '42'
+          ('42' as unknown) as sFunc
         )(1, 2)
       ).toThrow()
     })
 
     it('can be seeded with multiple arguments', () => {
-      const square = x => x * x
-      const add = (x, y) => x + y
+      const square = (x: number, _: number) => x * x
+      const add = (x: number, y: number) => x + y
       expect(
         compose(
           square,
@@ -103,9 +104,9 @@ describe('Utils', () => {
     })
 
     it('returns the first given argument if given no functions', () => {
-      expect(compose()(1, 2)).toBe(1)
+      expect(compose<number>()(1, 2)).toBe(1)
       expect(compose()(3)).toBe(3)
-      expect(compose()()).toBe(undefined)
+      expect(compose()(undefined)).toBe(undefined)
     })
 
     it('returns the first function if given only one', () => {
diff --git a/test/createStore.spec.js b/test/createStore.spec.ts
similarity index 90%
rename from test/createStore.spec.js
rename to test/createStore.spec.ts
index 6a837c0d1d..5e8ef1db50 100644
--- a/test/createStore.spec.js
+++ b/test/createStore.spec.ts
@@ -1,4 +1,11 @@
-import { createStore, combineReducers } from '../'
+import {
+  createStore,
+  combineReducers,
+  Reducer,
+  StoreEnhancer,
+  Action,
+  Store
+} from '..'
 import {
   addTodo,
   dispatchInMiddle,
@@ -9,7 +16,7 @@ import {
   unknownAction
 } from './helpers/actionCreators'
 import * as reducers from './helpers/reducers'
-import { from } from 'rxjs'
+import { from, ObservableInput } from 'rxjs'
 import { map } from 'rxjs/operators'
 import $$observable from 'symbol-observable'
 
@@ -26,11 +33,11 @@ describe('createStore', () => {
   })
 
   it('throws if reducer is not a function', () => {
-    expect(() => createStore()).toThrow()
+    expect(() => createStore(undefined)).toThrow()
 
-    expect(() => createStore('test')).toThrow()
+    expect(() => createStore(('test' as unknown) as Reducer)).toThrow()
 
-    expect(() => createStore({})).toThrow()
+    expect(() => createStore(({} as unknown) as Reducer)).toThrow()
 
     expect(() => createStore(() => {})).not.toThrow()
   })
@@ -568,13 +575,23 @@ describe('createStore', () => {
   })
 
   it('throws if enhancer is neither undefined nor a function', () => {
-    expect(() => createStore(reducers.todos, undefined, {})).toThrow()
+    expect(() =>
+      createStore(reducers.todos, undefined, ({} as unknown) as StoreEnhancer)
+    ).toThrow()
 
-    expect(() => createStore(reducers.todos, undefined, [])).toThrow()
+    expect(() =>
+      createStore(reducers.todos, undefined, ([] as unknown) as StoreEnhancer)
+    ).toThrow()
 
     expect(() => createStore(reducers.todos, undefined, null)).toThrow()
 
-    expect(() => createStore(reducers.todos, undefined, false)).toThrow()
+    expect(() =>
+      createStore(
+        reducers.todos,
+        undefined,
+        (false as unknown) as StoreEnhancer
+      )
+    ).toThrow()
 
     expect(() =>
       createStore(reducers.todos, undefined, undefined)
@@ -586,25 +603,27 @@ describe('createStore', () => {
 
     expect(() => createStore(reducers.todos, [])).not.toThrow()
 
-    expect(() => createStore(reducers.todos, {})).not.toThrow()
+    expect(() =>
+      createStore<{}, Action, {}, {}>(reducers.todos, {})
+    ).not.toThrow()
   })
 
   it('throws if nextReducer is not a function', () => {
     const store = createStore(reducers.todos)
 
-    expect(() => store.replaceReducer()).toThrow(
+    expect(() => store.replaceReducer(undefined)).toThrow(
       'Expected the nextReducer to be a function.'
     )
 
-    expect(() => store.replaceReducer(() => {})).not.toThrow()
+    expect(() => store.replaceReducer(() => [])).not.toThrow()
   })
 
   it('throws if listener is not a function', () => {
     const store = createStore(reducers.todos)
 
-    expect(() => store.subscribe()).toThrow()
+    expect(() => store.subscribe(undefined)).toThrow()
 
-    expect(() => store.subscribe('')).toThrow()
+    expect(() => store.subscribe(('' as unknown) as () => void)).toThrow()
 
     expect(() => store.subscribe(null)).toThrow()
 
@@ -654,11 +673,11 @@ describe('createStore', () => {
     })
 
     it('should pass an integration test with no unsubscribe', () => {
-      function foo(state = 0, action) {
+      function foo(state = 0, action: Action) {
         return action.type === 'foo' ? 1 : state
       }
 
-      function bar(state = 0, action) {
+      function bar(state = 0, action: Action) {
         return action.type === 'bar' ? 2 : state
       }
 
@@ -667,7 +686,7 @@ describe('createStore', () => {
       const results = []
 
       observable.subscribe({
-        next(state) {
+        next(state: any) {
           results.push(state)
         }
       })
@@ -683,11 +702,11 @@ describe('createStore', () => {
     })
 
     it('should pass an integration test with an unsubscribe', () => {
-      function foo(state = 0, action) {
+      function foo(state = 0, action: Action) {
         return action.type === 'foo' ? 1 : state
       }
 
-      function bar(state = 0, action) {
+      function bar(state = 0, action: Action) {
         return action.type === 'bar' ? 2 : state
       }
 
@@ -696,7 +715,7 @@ describe('createStore', () => {
       const results = []
 
       const sub = observable.subscribe({
-        next(state) {
+        next(state: any) {
           results.push(state)
         }
       })
@@ -709,25 +728,26 @@ describe('createStore', () => {
     })
 
     it('should pass an integration test with a common library (RxJS)', () => {
-      function foo(state = 0, action) {
+      function foo(state = 0, action: Action) {
         return action.type === 'foo' ? 1 : state
       }
 
-      function bar(state = 0, action) {
+      function bar(state = 0, action: Action) {
         return action.type === 'bar' ? 2 : state
       }
 
-      const store = createStore(combineReducers({ foo, bar }))
+      const store: ObservableInput<{ foo: number; bar: number }> = createStore(
+        combineReducers({ foo, bar })
+      )
       const observable = from(store)
       const results = []
 
       const sub = observable
         .pipe(map(state => ({ fromRx: true, ...state })))
         .subscribe(state => results.push(state))
-
-      store.dispatch({ type: 'foo' })
+      ;(store as Store).dispatch({ type: 'foo' })
       sub.unsubscribe()
-      store.dispatch({ type: 'bar' })
+      ;(store as Store).dispatch({ type: 'bar' })
 
       expect(results).toEqual([
         { foo: 0, bar: 0, fromRx: true },
@@ -742,10 +762,10 @@ describe('createStore', () => {
 
     const store = createStore(
       combineReducers({
-        x: (s = 0, a) => s,
+        x: (s = 0, _) => s,
         y: combineReducers({
-          z: (s = 0, a) => s,
-          w: (s = 0, a) => s
+          z: (s = 0, _) => s,
+          w: (s = 0, _) => s
         })
       })
     )
@@ -753,12 +773,12 @@ describe('createStore', () => {
     store.replaceReducer(
       combineReducers({
         y: combineReducers({
-          z: (s = 0, a) => s
+          z: (s = 0, _) => s
         })
       })
     )
 
-    expect(console.error.mock.calls.length).toBe(0)
+    expect((console.error as any).mock.calls.length).toBe(0)
     console.error = originalConsoleError
   })
 
@@ -766,15 +786,22 @@ describe('createStore', () => {
     const rootReducer = combineReducers(reducers)
     const dummyEnhancer = f => f
     expect(() =>
-      createStore(rootReducer, dummyEnhancer, dummyEnhancer)
+      // use a fake pre-loaded state to get a valid createStore signature
+      createStore(rootReducer, (dummyEnhancer as unknown) as {}, dummyEnhancer)
     ).toThrow()
   })
 
   it('throws if passing several enhancer functions with preloaded state', () => {
     const rootReducer = combineReducers(reducers)
-    const dummyEnhancer = f => f
+    const dummyEnhancer = (f: any) => f
     expect(() =>
-      createStore(rootReducer, { todos: [] }, dummyEnhancer, dummyEnhancer)
+      // work around the type so we can call this poorly
+      (createStore as any)(
+        rootReducer,
+        { todos: [] },
+        dummyEnhancer,
+        dummyEnhancer
+      )
     ).toThrow()
   })
 })
diff --git a/test/helpers/actionCreators.js b/test/helpers/actionCreators.ts
similarity index 52%
rename from test/helpers/actionCreators.js
rename to test/helpers/actionCreators.ts
index c75a7c3084..46964ea9ce 100644
--- a/test/helpers/actionCreators.js
+++ b/test/helpers/actionCreators.ts
@@ -7,13 +7,14 @@ import {
   THROW_ERROR,
   UNKNOWN_ACTION
 } from './actionTypes'
+import { Action, AnyAction, Dispatch } from '../..'
 
-export function addTodo(text) {
+export function addTodo(text: string): AnyAction {
   return { type: ADD_TODO, text }
 }
 
-export function addTodoAsync(text) {
-  return dispatch =>
+export function addTodoAsync(text: string) {
+  return (dispatch: Dispatch): Promise<void> =>
     new Promise(resolve =>
       setImmediate(() => {
         dispatch(addTodo(text))
@@ -22,49 +23,49 @@ export function addTodoAsync(text) {
     )
 }
 
-export function addTodoIfEmpty(text) {
-  return (dispatch, getState) => {
+export function addTodoIfEmpty(text: string) {
+  return (dispatch: Dispatch, getState: () => any) => {
     if (!getState().length) {
       dispatch(addTodo(text))
     }
   }
 }
 
-export function dispatchInMiddle(boundDispatchFn) {
+export function dispatchInMiddle(boundDispatchFn: () => void): AnyAction {
   return {
     type: DISPATCH_IN_MIDDLE,
     boundDispatchFn
   }
 }
 
-export function getStateInMiddle(boundGetStateFn) {
+export function getStateInMiddle(boundGetStateFn: () => void): AnyAction {
   return {
     type: GET_STATE_IN_MIDDLE,
     boundGetStateFn
   }
 }
 
-export function subscribeInMiddle(boundSubscribeFn) {
+export function subscribeInMiddle(boundSubscribeFn: () => void): AnyAction {
   return {
     type: SUBSCRIBE_IN_MIDDLE,
     boundSubscribeFn
   }
 }
 
-export function unsubscribeInMiddle(boundUnsubscribeFn) {
+export function unsubscribeInMiddle(boundUnsubscribeFn: () => void): AnyAction {
   return {
     type: UNSUBSCRIBE_IN_MIDDLE,
     boundUnsubscribeFn
   }
 }
 
-export function throwError() {
+export function throwError(): Action {
   return {
     type: THROW_ERROR
   }
 }
 
-export function unknownAction() {
+export function unknownAction(): Action {
   return {
     type: UNKNOWN_ACTION
   }
diff --git a/test/helpers/actionTypes.js b/test/helpers/actionTypes.ts
similarity index 100%
rename from test/helpers/actionTypes.js
rename to test/helpers/actionTypes.ts
diff --git a/test/helpers/middleware.js b/test/helpers/middleware.js
deleted file mode 100644
index 06f074fffa..0000000000
--- a/test/helpers/middleware.js
+++ /dev/null
@@ -1,4 +0,0 @@
-export function thunk({ dispatch, getState }) {
-  return next => action =>
-    typeof action === 'function' ? action(dispatch, getState) : next(action)
-}
diff --git a/test/helpers/middleware.ts b/test/helpers/middleware.ts
new file mode 100644
index 0000000000..5bf51a6754
--- /dev/null
+++ b/test/helpers/middleware.ts
@@ -0,0 +1,12 @@
+import { MiddlewareAPI, Dispatch, AnyAction } from '../..'
+
+type ThunkAction<T extends any = any> = T extends AnyAction
+  ? AnyAction
+  : T extends Function
+  ? T
+  : never
+
+export function thunk({ dispatch, getState }: MiddlewareAPI) {
+  return (next: Dispatch) => <T>(action: ThunkAction) =>
+    typeof action === 'function' ? action(dispatch, getState) : next(action)
+}
diff --git a/test/helpers/reducers.js b/test/helpers/reducers.ts
similarity index 56%
rename from test/helpers/reducers.js
rename to test/helpers/reducers.ts
index f0030fb2dd..c10d3d017e 100644
--- a/test/helpers/reducers.js
+++ b/test/helpers/reducers.ts
@@ -6,14 +6,21 @@ import {
   UNSUBSCRIBE_IN_MIDDLE,
   THROW_ERROR
 } from './actionTypes'
+import { AnyAction } from '../..'
 
-function id(state = []) {
+function id(state: { id: number }[] = []) {
   return (
     state.reduce((result, item) => (item.id > result ? item.id : result), 0) + 1
   )
 }
 
-export function todos(state = [], action) {
+export interface Todo {
+  id: number
+  text: string
+}
+export type TodoAction = { type: 'ADD_TODO'; text: string } | AnyAction
+
+export function todos(state: Todo[] = [], action: TodoAction) {
   switch (action.type) {
     case ADD_TODO:
       return [
@@ -28,7 +35,7 @@ export function todos(state = [], action) {
   }
 }
 
-export function todosReverse(state = [], action) {
+export function todosReverse(state: Todo[] = [], action: TodoAction) {
   switch (action.type) {
     case ADD_TODO:
       return [
@@ -43,7 +50,12 @@ export function todosReverse(state = [], action) {
   }
 }
 
-export function dispatchInTheMiddleOfReducer(state = [], action) {
+export function dispatchInTheMiddleOfReducer(
+  state = [],
+  action:
+    | { type: 'DISPATCH_IN_MIDDLE'; boundDispatchFn: () => void }
+    | AnyAction
+) {
   switch (action.type) {
     case DISPATCH_IN_MIDDLE:
       action.boundDispatchFn()
@@ -53,7 +65,12 @@ export function dispatchInTheMiddleOfReducer(state = [], action) {
   }
 }
 
-export function getStateInTheMiddleOfReducer(state = [], action) {
+export function getStateInTheMiddleOfReducer(
+  state = [],
+  action:
+    | { type: 'DISPATCH_IN_MIDDLE'; boundGetStateFn: () => void }
+    | AnyAction
+) {
   switch (action.type) {
     case GET_STATE_IN_MIDDLE:
       action.boundGetStateFn()
@@ -63,7 +80,12 @@ export function getStateInTheMiddleOfReducer(state = [], action) {
   }
 }
 
-export function subscribeInTheMiddleOfReducer(state = [], action) {
+export function subscribeInTheMiddleOfReducer(
+  state = [],
+  action:
+    | { type: 'DISPATCH_IN_MIDDLE'; boundSubscribeFn: () => void }
+    | AnyAction
+) {
   switch (action.type) {
     case SUBSCRIBE_IN_MIDDLE:
       action.boundSubscribeFn()
@@ -73,7 +95,12 @@ export function subscribeInTheMiddleOfReducer(state = [], action) {
   }
 }
 
-export function unsubscribeInTheMiddleOfReducer(state = [], action) {
+export function unsubscribeInTheMiddleOfReducer(
+  state = [],
+  action:
+    | { type: 'DISPATCH_IN_MIDDLE'; boundUnsubscribeFn: () => void }
+    | AnyAction
+) {
   switch (action.type) {
     case UNSUBSCRIBE_IN_MIDDLE:
       action.boundUnsubscribeFn()
@@ -83,7 +110,7 @@ export function unsubscribeInTheMiddleOfReducer(state = [], action) {
   }
 }
 
-export function errorThrowingReducer(state = [], action) {
+export function errorThrowingReducer(state = [], action: AnyAction) {
   switch (action.type) {
     case THROW_ERROR:
       throw new Error()
diff --git a/test/replaceReducers.spec.ts b/test/replaceReducers.spec.ts
new file mode 100644
index 0000000000..ed70106b12
--- /dev/null
+++ b/test/replaceReducers.spec.ts
@@ -0,0 +1,18 @@
+import { createStore, combineReducers } from '..'
+
+describe('replaceReducers test', () => {
+  it('returns the original store', () => {
+    const nextReducer = combineReducers({
+      foo: (state = 1, _action) => state,
+      bar: (state = 2, _action) => state
+    })
+    const store = createStore((state, action) => {
+      if (state === undefined) return { type: 5 }
+      return action
+    })
+
+    const nextStore = store.replaceReducer(nextReducer)
+
+    expect(nextStore).toBe(store)
+  })
+})
diff --git a/test/typescript.spec.js b/test/typescript.spec.ts
similarity index 100%
rename from test/typescript.spec.js
rename to test/typescript.spec.ts
diff --git a/test/typescript/.eslintrc.js b/test/typescript/.eslintrc.js
new file mode 100644
index 0000000000..d94f0a7925
--- /dev/null
+++ b/test/typescript/.eslintrc.js
@@ -0,0 +1,7 @@
+module.exports = {
+  rules: {
+    'no-unused-expressions': 'off',
+    '@typescript-eslint/no-namespace': 'off',
+    '@typescript-eslint/no-unused-vars': 'off'
+  }
+}
diff --git a/test/typescript/actionCreators.ts b/test/typescript/actionCreators.ts
index e3594a0712..f7a15bcd75 100644
--- a/test/typescript/actionCreators.ts
+++ b/test/typescript/actionCreators.ts
@@ -4,7 +4,7 @@ import {
   Dispatch,
   bindActionCreators,
   ActionCreatorsMapObject
-} from 'redux'
+} from '../..'
 
 interface AddTodoAction extends Action {
   text: string
diff --git a/test/typescript/actions.ts b/test/typescript/actions.ts
index 1b701ce77d..480a34151b 100644
--- a/test/typescript/actions.ts
+++ b/test/typescript/actions.ts
@@ -1,4 +1,4 @@
-import { Action as ReduxAction } from 'redux'
+import { Action as ReduxAction } from '../..'
 
 namespace FSA {
   interface Action<P> extends ReduxAction {
diff --git a/test/typescript/compose.ts b/test/typescript/compose.ts
index ae2fc5b32d..cfc608bad3 100644
--- a/test/typescript/compose.ts
+++ b/test/typescript/compose.ts
@@ -1,4 +1,4 @@
-import { compose } from 'redux'
+import { compose } from '../..'
 
 // adapted from DefinitelyTyped/compose-function
 
diff --git a/test/typescript/dispatch.ts b/test/typescript/dispatch.ts
index 72f8998856..9cb125e37a 100644
--- a/test/typescript/dispatch.ts
+++ b/test/typescript/dispatch.ts
@@ -1,4 +1,4 @@
-import { Dispatch, AnyAction } from 'redux'
+import { Dispatch } from '../..'
 
 /**
  * Default Dispatch type accepts any object with `type` property.
diff --git a/test/typescript/enhancers.ts b/test/typescript/enhancers.ts
index 7259b4884b..c6d29af6a0 100644
--- a/test/typescript/enhancers.ts
+++ b/test/typescript/enhancers.ts
@@ -1,5 +1,4 @@
-import { PreloadedState } from '../../index'
-import { StoreEnhancer, Action, AnyAction, Reducer, createStore } from 'redux'
+import { StoreEnhancer, Action, AnyAction, Reducer, createStore } from '../..'
 
 interface State {
   someField: 'string'
@@ -12,7 +11,25 @@ const reducer: Reducer<State> = null as any
 function dispatchExtension() {
   type PromiseDispatch = <T extends Action>(promise: Promise<T>) => Promise<T>
 
-  const enhancer: StoreEnhancer<{ dispatch: PromiseDispatch }> = null as any
+  const enhancer: StoreEnhancer<{
+    dispatch: PromiseDispatch
+  }> = createStore => <S, A extends Action = AnyAction>(
+    reducer: Reducer<S, A>,
+    preloadedState?: any
+  ) => {
+    const store = createStore(reducer, preloadedState)
+    return {
+      ...store,
+      dispatch: (action: any) => {
+        if (action.type) {
+          store.dispatch(action)
+        } else if (action.then) {
+          action.then(store.dispatch)
+        }
+        return action
+      }
+    }
+  }
 
   const store = createStore(reducer, enhancer)
 
@@ -37,10 +54,21 @@ function stateExtension() {
     A extends Action = AnyAction
   >(
     reducer: Reducer<S, A>,
-    preloadedState?: PreloadedState<S>
+    preloadedState?: any
   ) => {
-    const wrappedReducer: Reducer<S & ExtraState, A> = null as any
-    const wrappedPreloadedState: PreloadedState<S & ExtraState> = null as any
+    const wrappedReducer: Reducer<S & ExtraState, A> = (state, action) => {
+      const newState = reducer(state, action)
+      return {
+        ...newState,
+        extraField: 'extra'
+      }
+    }
+    const wrappedPreloadedState = preloadedState
+      ? {
+          ...preloadedState,
+          extraField: 'extra'
+        }
+      : undefined
     return createStore(wrappedReducer, wrappedPreloadedState)
   }
 
@@ -65,3 +93,198 @@ function extraMethods() {
   // typings:expect-error
   store.wrongMethod()
 }
+
+/**
+ * replaceReducer with a store enhancer
+ */
+function replaceReducerExtender() {
+  interface ExtraState {
+    extraField: 'extra'
+  }
+
+  const enhancer: StoreEnhancer<
+    { method(): string },
+    ExtraState
+  > = createStore => <S, A extends Action = AnyAction>(
+    reducer: Reducer<S, A>,
+    preloadedState?: any
+  ) => {
+    const wrappedReducer: Reducer<S & ExtraState, A> = (state, action) => {
+      const newState = reducer(state, action)
+      return {
+        ...newState,
+        extraField: 'extra'
+      }
+    }
+    const wrappedPreloadedState = preloadedState
+      ? {
+          ...preloadedState,
+          extraField: 'extra'
+        }
+      : undefined
+    return createStore(wrappedReducer, wrappedPreloadedState)
+  }
+
+  const store = createStore(reducer, enhancer)
+
+  const newReducer = (
+    state: { test: boolean } = { test: true },
+    _: AnyAction
+  ) => state
+
+  const newStore = store.replaceReducer(newReducer)
+  newStore.getState().test
+  newStore.getState().extraField
+  // typings:expect-error
+  newStore.getState().wrongField
+
+  const res: string = newStore.method()
+  // typings:expect-error
+  newStore.wrongMethod()
+}
+
+function mhelmersonExample() {
+  interface State {
+    someField: 'string'
+  }
+
+  interface ExtraState {
+    extraField: 'extra'
+  }
+
+  const reducer: Reducer<State> = null as any
+
+  function stateExtensionExpectedToWork() {
+    interface ExtraState {
+      extraField: 'extra'
+    }
+
+    const enhancer: StoreEnhancer<{}, ExtraState> = createStore => <
+      S,
+      A extends Action = AnyAction
+    >(
+      reducer: Reducer<S, A>,
+      preloadedState?: any
+    ) => {
+      const wrappedReducer: Reducer<S & ExtraState, A> = (state, action) => {
+        const newState = reducer(state, action)
+        return {
+          ...newState,
+          extraField: 'extra'
+        }
+      }
+      const wrappedPreloadedState = preloadedState
+        ? {
+            ...preloadedState,
+            extraField: 'extra'
+          }
+        : undefined
+      const store = createStore(wrappedReducer, wrappedPreloadedState)
+      return {
+        ...store,
+        replaceReducer<NS, NA extends Action = AnyAction>(
+          nextReducer: (
+            state: NS & ExtraState | undefined,
+            action: NA
+          ) => NS & ExtraState
+        ) {
+          const nextWrappedReducer: Reducer<NS & ExtraState, NA> = (
+            state,
+            action
+          ) => {
+            const newState = nextReducer(state, action)
+            return {
+              ...newState,
+              extraField: 'extra'
+            }
+          }
+          return store.replaceReducer(nextWrappedReducer)
+        }
+      }
+    }
+
+    const store = createStore(reducer, enhancer)
+    store.replaceReducer(reducer)
+
+    store.getState().extraField
+    // typings:expect-error
+    store.getState().wrongField
+    // typings:expect-error
+    store.getState().test
+
+    const newReducer = (
+      state: { test: boolean } = { test: true },
+      _: AnyAction
+    ) => state
+
+    const newStore = store.replaceReducer(newReducer)
+    newStore.getState().test
+    newStore.getState().extraField
+    // typings:expect-error
+    newStore.getState().wrongField
+  }
+}
+
+function finalHelmersonExample() {
+  interface ExtraState {
+    foo: string
+  }
+
+  function persistReducer<S, A extends AnyAction>(
+    config: any,
+    reducer: Reducer<S, A>
+  ) {
+    return (state: S & ExtraState | undefined, action: AnyAction) => {
+      const newState = reducer(state, (action as unknown) as A)
+      return {
+        ...newState,
+        foo: 'hi'
+      }
+    }
+  }
+
+  function persistStore<S>(store: S) {
+    return store
+  }
+
+  function createPersistEnhancer(
+    persistConfig: any
+  ): StoreEnhancer<{}, ExtraState> {
+    return createStore => <S, A extends Action = AnyAction>(
+      reducer: Reducer<S, A>,
+      preloadedState?: any
+    ) => {
+      const persistedReducer = persistReducer<S, A>(persistConfig, reducer)
+      const store = createStore(persistedReducer, preloadedState)
+      const persistor = persistStore(store)
+
+      return {
+        ...store,
+        replaceReducer: nextReducer => {
+          return store.replaceReducer(
+            persistReducer(persistConfig, nextReducer)
+          )
+        },
+        persistor
+      }
+    }
+  }
+
+  const store = createStore(reducer, createPersistEnhancer('hi'))
+
+  store.getState().foo
+  // typings:expect-error
+  store.getState().wrongField
+
+  const newReducer = (
+    state: { test: boolean } = { test: true },
+    _: AnyAction
+  ) => state
+
+  const newStore = store.replaceReducer(newReducer)
+  newStore.getState().test
+  // typings:expect-error
+  newStore.getState().whatever
+  // typings:expect-error
+  newStore.getState().wrongField
+}
diff --git a/test/typescript/injectedDispatch.ts b/test/typescript/injectedDispatch.ts
index f115b699fe..46dc9a7016 100644
--- a/test/typescript/injectedDispatch.ts
+++ b/test/typescript/injectedDispatch.ts
@@ -1,4 +1,4 @@
-import { Dispatch, Action } from 'redux'
+import { Dispatch, Action } from '../..'
 
 interface Component<P> {
   props: P
diff --git a/test/typescript/middleware.ts b/test/typescript/middleware.ts
index 7b1c48443c..a0f9afa7f6 100644
--- a/test/typescript/middleware.ts
+++ b/test/typescript/middleware.ts
@@ -2,13 +2,12 @@ import {
   Middleware,
   MiddlewareAPI,
   applyMiddleware,
-  StoreEnhancer,
   createStore,
   Dispatch,
   Reducer,
   Action,
   AnyAction
-} from 'redux'
+} from '../..'
 
 /**
  * Logger middleware doesn't add any extra types to dispatch, just logs actions
diff --git a/test/typescript/reducers.ts b/test/typescript/reducers.ts
index 5a109eeece..f013f83b37 100644
--- a/test/typescript/reducers.ts
+++ b/test/typescript/reducers.ts
@@ -1,4 +1,4 @@
-import { Reducer, Action, combineReducers, ReducersMapObject } from 'redux'
+import { Reducer, Action, combineReducers, ReducersMapObject } from '../..'
 
 /**
  * Simple reducer definition with no action shape checks.
diff --git a/test/typescript/replaceReducer.ts b/test/typescript/replaceReducer.ts
new file mode 100644
index 0000000000..03b5b7fd90
--- /dev/null
+++ b/test/typescript/replaceReducer.ts
@@ -0,0 +1,26 @@
+import { combineReducers, createStore } from '../..'
+
+/**
+ * verify that replaceReducer maintains strict typing if the new types change
+ */
+const bar = (state = { value: 'bar' }) => state
+const baz = (state = { value: 'baz' }) => state
+const ACTION = {
+  type: 'action'
+}
+
+const originalCompositeReducer = combineReducers({ bar })
+const store = createStore(originalCompositeReducer)
+store.dispatch(ACTION)
+
+const firstState = store.getState()
+firstState.bar.value
+// typings:expect-error
+firstState.baz.value
+
+const nextStore = store.replaceReducer(combineReducers({ baz })) // returns ->  { baz: { value: 'baz' }}
+
+const nextState = nextStore.getState()
+// typings:expect-error
+nextState.bar.value
+nextState.baz.value
diff --git a/test/typescript/store.ts b/test/typescript/store.ts
index 123464ce55..dd7d76296e 100644
--- a/test/typescript/store.ts
+++ b/test/typescript/store.ts
@@ -4,11 +4,10 @@ import {
   Reducer,
   Action,
   StoreEnhancer,
-  StoreCreator,
-  StoreEnhancerStoreCreator,
   Unsubscribe,
-  Observer
-} from 'redux'
+  Observer,
+  ExtendState
+} from '../..'
 import 'symbol-observable'
 
 type State = {
@@ -19,6 +18,41 @@ type State = {
   }
 }
 
+/* extended state */
+const noExtend: ExtendState<State, never> = {
+  a: 'a',
+  b: {
+    c: 'c',
+    d: 'd'
+  }
+}
+// typings:expect-error
+const noExtendError: ExtendState<State, never> = {
+  a: 'a',
+  b: {
+    c: 'c',
+    d: 'd'
+  },
+  e: 'oops'
+}
+
+const yesExtend: ExtendState<State, { yes: 'we can' }> = {
+  a: 'a',
+  b: {
+    c: 'c',
+    d: 'd'
+  },
+  yes: 'we can'
+}
+// typings:expect-error
+const yesExtendError: ExtendState<State, { yes: 'we can' }> = {
+  a: 'a',
+  b: {
+    c: 'c',
+    d: 'd'
+  }
+}
+
 interface DerivedAction extends Action {
   type: 'a'
   b: 'b'
@@ -56,6 +90,9 @@ const funcWithStore = (store: Store<State, DerivedAction>) => {}
 
 const store: Store<State> = createStore(reducer)
 
+// ensure that an array-based state works
+const arrayReducer = (state: any[] = []) => state || []
+const storeWithArrayState: Store<any[]> = createStore(arrayReducer)
 const storeWithPreloadedState: Store<State> = createStore(reducer, {
   a: 'a',
   b: { c: 'c', d: 'd' }
diff --git a/test/typescript/tsconfig.json b/test/typescript/tsconfig.json
index be426fdcac..9af8c36a48 100644
--- a/test/typescript/tsconfig.json
+++ b/test/typescript/tsconfig.json
@@ -1,10 +1,7 @@
 {
   "compilerOptions": {
-    "lib": ["es2015", "dom"],
+    "lib": ["es2017", "dom"],
     "strict": true,
-    "baseUrl": "../..",
-    "paths": {
-      "redux": ["index.d.ts"]
-    }
+    "baseUrl": "../.."
   }
 }
diff --git a/test/utils/isPlainObject.spec.js b/test/utils/isPlainObject.spec.ts
similarity index 93%
rename from test/utils/isPlainObject.spec.js
rename to test/utils/isPlainObject.spec.ts
index ddb7775b46..492ed6d38d 100644
--- a/test/utils/isPlainObject.spec.js
+++ b/test/utils/isPlainObject.spec.ts
@@ -16,7 +16,7 @@ describe('isPlainObject', () => {
     expect(isPlainObject(new Date())).toBe(false)
     expect(isPlainObject([1, 2, 3])).toBe(false)
     expect(isPlainObject(null)).toBe(false)
-    expect(isPlainObject()).toBe(false)
+    expect(isPlainObject(undefined)).toBe(false)
     expect(isPlainObject({ x: 1, y: 2 })).toBe(true)
   })
 })
diff --git a/test/utils/warning.spec.js b/test/utils/warning.spec.ts
similarity index 100%
rename from test/utils/warning.spec.js
rename to test/utils/warning.spec.ts
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000000..1ef2e9f083
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,69 @@
+{
+  "compilerOptions": {
+    /* Basic Options */
+    "target": "esnext" /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017','ES2018' or 'ESNEXT'. */,
+    "module": "esnext" /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */,
+    "lib": [
+      "dom",
+      "es2017"
+    ] /* Specify library files to be included in the compilation. */,
+    // "allowJs": true /* Allow javascript files to be compiled. */,
+    // "checkJs": true,                       /* Report errors in .js files. */
+    "jsx": "react" /* Specify JSX code generation: 'preserve', 'react-native', or 'react'. */,
+    "declaration": true /* Generates corresponding '.d.ts' file. */,
+    // "declarationMap": true /* Generates a sourcemap for each corresponding '.d.ts' file. */,
+    "declarationDir": "./types" /* Output directory for generated declaration files. */,
+    // "emitDeclarationOnly": true /* Only emit ‘.d.ts’ declaration files. */,
+    "sourceMap": true /* Generates corresponding '.map' file. */,
+    // "outFile": "./",                       /* Concatenate and emit output to single file. */
+    // "outDir": "./types" /* Redirect output structure to the directory. */,
+    // "rootDir": "./",                       /* Specify the root directory of input files. Use to control the output directory structure with --outDir. */
+    // "composite": true,                     /* Enable project compilation */
+    "removeComments": false /* Do not emit comments to output. */,
+    // "noEmit": true /* Do not emit outputs. */,
+    // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
+    // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
+    // "isolatedModules": true /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */,
+
+    /* Strict Type-Checking Options */
+    "strict": true /* Enable all strict type-checking options. */,
+    // "noImplicitAny": true,                 /* Raise error on expressions and declarations with an implied 'any' type. */
+    // "strictNullChecks": true,              /* Enable strict null checks. */
+    // "strictFunctionTypes": true,           /* Enable strict checking of function types. */
+    // "strictBindCallApply": true,           /* Enable strict 'bind', 'call', and 'apply' methods on functions. */
+    // "strictPropertyInitialization": true,  /* Enable strict checking of property initialization in classes. */
+    // "noImplicitThis": true,                /* Raise error on 'this' expressions with an implied 'any' type. */
+    // "alwaysStrict": true,                  /* Parse in strict mode and emit "use strict" for each source file. */
+
+    /* Additional Checks */
+    "noUnusedLocals": true /* Report errors on unused locals. */,
+    "noUnusedParameters": true /* Report errors on unused parameters. */,
+    // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
+    // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+
+    /* Module Resolution Options */
+    "moduleResolution": "node" /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */,
+    "baseUrl": "./" /* Base directory to resolve non-absolute module names. */,
+    // "paths": {
+    //   "*": ["*", "types/*"]
+    // } /* A series of entries which re-map imports to lookup locations relative to the 'baseUrl'. */,
+    // "rootDirs": [],                        /* List of root folders whose combined content represents the structure of the project at runtime. */
+    // "typeRoots": [],                       /* List of folders to include type definitions from. */
+    // "types": [],                           /* Type declaration files to be included in compilation. */
+    // "allowSyntheticDefaultImports": true,  /* Allow default imports from modules with no default export. This does not affect code emit, just typechecking. */
+    "esModuleInterop": true /* Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'. */,
+    // "preserveSymlinks": true,              /* Do not resolve the real path of symlinks. */
+
+    /* Source Map Options */
+    // "sourceRoot": "",                      /* Specify the location where debugger should locate TypeScript files instead of source locations. */
+    // "mapRoot": "",                         /* Specify the location where debugger should locate map files instead of generated locations. */
+    // "inlineSourceMap": true,               /* Emit a single file with source maps instead of having a separate file. */
+    // "inlineSources": true,                 /* Emit the source alongside the sourcemaps within a single file; requires '--inlineSourceMap' or '--sourceMap' to be set. */
+
+    /* Experimental Options */
+    // "experimentalDecorators": true,        /* Enables experimental support for ES7 decorators. */
+    // "emitDecoratorMetadata": true,         /* Enables experimental support for emitting type metadata for decorators. */
+    "skipLibCheck": true /* This trusts that typings files (@types) are accurate and don't need to be checked by the compiler. This speeds build times. */
+  },
+  "include": ["./src/**/*"]
+}