diff --git a/.gitattributes b/.gitattributes
index 21be40ca..94a1c394 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1,6 +1,8 @@
 /.gitattributes export-ignore
 /.github/ export-ignore
 /.gitignore export-ignore
+/phpstan.neon.dist export-ignore
 /phpunit.xml.dist export-ignore
 /phpunit.xml.legacy export-ignore
 /tests/ export-ignore
+/types/ export-ignore
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e20fa07c..749a5e87 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -45,3 +45,18 @@ jobs:
       - run: composer self-update --2.2 # downgrade Composer for HHVM
       - run: hhvm $(which composer) install
       - run: hhvm vendor/bin/phpunit
+
+  PHPStan:
+    name: PHPStan
+    runs-on: ubuntu-20.04
+    strategy:
+      matrix:
+        php:
+          - 8.1
+    steps:
+      - uses: actions/checkout@v3
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+      - run: composer require phpstan/phpstan
+      - run: vendor/bin/phpstan
diff --git a/phpstan.neon.dist b/phpstan.neon.dist
new file mode 100644
index 00000000..609d438c
--- /dev/null
+++ b/phpstan.neon.dist
@@ -0,0 +1,4 @@
+parameters:
+  paths:
+    - types
+  level: max
diff --git a/src/PromiseInterface.php b/src/PromiseInterface.php
index edcb0077..f52f5443 100644
--- a/src/PromiseInterface.php
+++ b/src/PromiseInterface.php
@@ -2,6 +2,9 @@
 
 namespace React\Promise;
 
+/**
+ * @template T
+ */
 interface PromiseInterface
 {
     /**
@@ -32,10 +35,11 @@ interface PromiseInterface
      *      than once.
      *  3. `$onProgress` (deprecated) may be called multiple times.
      *
-     * @param callable|null $onFulfilled
+     * @template TReturn of mixed
+     * @param callable(T): TReturn $onFulfilled
      * @param callable|null $onRejected
      * @param callable|null $onProgress This argument is deprecated and should not be used anymore.
-     * @return PromiseInterface
+     * @return (TReturn is PromiseInterface ? TReturn : PromiseInterface<TReturn>)
      */
     public function then(callable $onFulfilled = null, callable $onRejected = null, callable $onProgress = null);
 }
diff --git a/src/functions.php b/src/functions.php
index 429f0e73..469fedc4 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -13,8 +13,9 @@
  *
  * If `$promiseOrValue` is a promise, it will be returned as is.
  *
- * @param mixed $promiseOrValue
- * @return PromiseInterface
+ * @template T
+ * @param T $promiseOrValue
+ * @return PromiseInterface<T>
  */
 function resolve($promiseOrValue = null)
 {
diff --git a/types/PromiseInterface.php b/types/PromiseInterface.php
new file mode 100644
index 00000000..5fd5e8d8
--- /dev/null
+++ b/types/PromiseInterface.php
@@ -0,0 +1,9 @@
+<?php
+
+use function PHPStan\Testing\assertType;
+use function React\Promise\resolve;
+
+$passThroughBoolFn = static fn (bool $bool): bool => $bool;
+
+assertType('React\Promise\PromiseInterface<bool>', resolve(true));
+assertType('React\Promise\PromiseInterface<bool>', resolve(true)->then($passThroughBoolFn));