diff --git a/lib/AST/Decl.cpp b/lib/AST/Decl.cpp
index 0df71e8715055..f325fec44f60b 100644
--- a/lib/AST/Decl.cpp
+++ b/lib/AST/Decl.cpp
@@ -1869,11 +1869,18 @@ const DeclContext *
 ValueDecl::getFormalAccessScope(const DeclContext *useDC) const {
   const DeclContext *result = getDeclContext();
   Accessibility access = getFormalAccess(useDC);
+  bool swift3PrivateChecked = false;
 
   while (!result->isModuleScopeContext()) {
     if (result->isLocalContext())
       return result;
 
+    if (access == Accessibility::Private && !swift3PrivateChecked) {
+      if (result->getASTContext().LangOpts.EnableSwift3Private)
+        return result;
+      swift3PrivateChecked = true;
+    }
+
     if (auto enclosingNominal = dyn_cast<NominalTypeDecl>(result)) {
       access = std::min(access, enclosingNominal->getFormalAccess(useDC));
 
@@ -1895,7 +1902,6 @@ ValueDecl::getFormalAccessScope(const DeclContext *useDC) const {
 
   switch (access) {
   case Accessibility::Private:
-    // TODO: Implement 'private'
   case Accessibility::FilePrivate:
     assert(result->isModuleScopeContext());
     return result;
diff --git a/lib/AST/NameLookup.cpp b/lib/AST/NameLookup.cpp
index b42bf7bb4ed81..a64b99a618b5c 100644
--- a/lib/AST/NameLookup.cpp
+++ b/lib/AST/NameLookup.cpp
@@ -1017,9 +1017,12 @@ static bool checkAccessibility(const DeclContext *useDC,
   if (!useDC)
     return access == Accessibility::Public;
 
+  assert(sourceDC && "ValueDecl being accessed must have a valid DeclContext");
   switch (access) {
   case Accessibility::Private:
-    // TODO: Implement 'private'.
+    if (sourceDC->getASTContext().LangOpts.EnableSwift3Private)
+      return useDC == sourceDC || useDC->isChildContextOf(sourceDC);
+    SWIFT_FALLTHROUGH;
   case Accessibility::FilePrivate:
     return useDC->getModuleScopeContext() == sourceDC->getModuleScopeContext();
   case Accessibility::Internal: {
diff --git a/lib/Sema/TypeCheckDecl.cpp b/lib/Sema/TypeCheckDecl.cpp
index 1a65b1370f169..96b6e278603d8 100644
--- a/lib/Sema/TypeCheckDecl.cpp
+++ b/lib/Sema/TypeCheckDecl.cpp
@@ -6168,7 +6168,9 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
         const DeclContext *desiredAccessScope;
         switch (AA->getAccess()) {
         case Accessibility::Private:
-          // TODO: Implement 'private'.
+          assert(ED->getDeclContext()->isModuleScopeContext() &&
+                 "non-top-level extensions make 'private' != 'fileprivate'");
+          SWIFT_FALLTHROUGH;
         case Accessibility::FilePrivate:
           desiredAccessScope = ED->getModuleScopeContext();
           break;
diff --git a/stdlib/public/SDK/Foundation/IndexPath.swift b/stdlib/public/SDK/Foundation/IndexPath.swift
index 0aa210aa00441..a61ab67cff965 100644
--- a/stdlib/public/SDK/Foundation/IndexPath.swift
+++ b/stdlib/public/SDK/Foundation/IndexPath.swift
@@ -26,7 +26,7 @@ public struct IndexPath : ReferenceConvertible, Equatable, Hashable, MutableColl
     public typealias Index = Array<Int>.Index
     public typealias Indices = DefaultRandomAccessIndices<IndexPath>
     
-    private var _indexes : Array<Int>
+    fileprivate var _indexes : Array<Int>
     
     /// Initialize an empty index path.
     public init() {
diff --git a/test/Sema/accessibility_private.swift b/test/Sema/accessibility_private.swift
new file mode 100644
index 0000000000000..a2ab83cc08345
--- /dev/null
+++ b/test/Sema/accessibility_private.swift
@@ -0,0 +1,112 @@
+// RUN: %target-parse-verify-swift
+
+class Container {
+  private func foo() {} // expected-note * {{declared here}}
+  private var bar = 0 // expected-note * {{declared here}}
+
+  private struct PrivateInner {} // expected-note * {{declared here}}
+
+  func localTest() {
+    foo()
+    self.foo()
+
+    _ = bar
+    bar = 5
+    _ = self.bar
+    self.bar = 5
+
+    privateExtensionMethod() // FIXME expected-error {{use of unresolved identifier 'privateExtensionMethod'}}
+    self.privateExtensionMethod() // expected-error {{'privateExtensionMethod' is inaccessible due to 'private' protection level}}
+
+    _ = PrivateInner()
+    _ = Container.PrivateInner()
+  }
+
+  struct Inner {
+    func test(obj: Container) {
+      obj.foo()
+      _ = obj.bar
+      obj.bar = 5
+      obj.privateExtensionMethod() // expected-error {{'privateExtensionMethod' is inaccessible due to 'private' protection level}}
+
+      _ = PrivateInner()
+      _ = Container.PrivateInner()
+    }
+
+    var inner: PrivateInner? // expected-error {{property must be declared private because its type uses a private type}}
+    var innerQualified: Container.PrivateInner? // expected-error {{property must be declared private because its type uses a private type}}
+  }
+
+  var inner: PrivateInner? // expected-error {{property must be declared private because its type uses a private type}}
+  var innerQualified: Container.PrivateInner? // expected-error {{property must be declared private because its type uses a private type}}
+}
+
+func test(obj: Container) {
+  obj.foo() // expected-error {{'foo' is inaccessible due to 'private' protection level}}
+  _ = obj.bar // expected-error {{'bar' is inaccessible due to 'private' protection level}}
+  obj.bar = 5 // expected-error {{'bar' is inaccessible due to 'private' protection level}}
+  obj.privateExtensionMethod() // expected-error {{'privateExtensionMethod' is inaccessible due to 'private' protection level}}
+
+  _ = Container.PrivateInner() // expected-error {{'PrivateInner' is inaccessible due to 'private' protection level}}
+}
+
+extension Container {
+  private func privateExtensionMethod() {} // expected-note * {{declared here}}
+
+  func extensionTest() {
+    foo() // FIXME expected-error {{use of unresolved identifier 'foo'}}
+    self.foo() // expected-error {{'foo' is inaccessible due to 'private' protection level}}
+
+    _ = bar // FIXME expected-error {{use of unresolved identifier 'bar'}}
+    bar = 5 // FIXME expected-error {{use of unresolved identifier 'bar'}}
+    _ = self.bar // expected-error {{'bar' is inaccessible due to 'private' protection level}}
+    self.bar = 5 // expected-error {{'bar' is inaccessible due to 'private' protection level}}
+
+    privateExtensionMethod()
+    self.privateExtensionMethod()
+
+    _ = PrivateInner() // FIXME expected-error {{use of unresolved identifier 'PrivateInner'}}
+    _ = Container.PrivateInner() // expected-error {{'PrivateInner' is inaccessible due to 'private' protection level}}
+  }
+
+  // FIXME: Why do these errors happen twice?
+  var extensionInner: PrivateInner? { return nil } // FIXME expected-error 2 {{use of undeclared type 'PrivateInner'}}
+  var extensionInnerQualified: Container.PrivateInner? { return nil } // FIXME expected-error 2 {{'PrivateInner' is not a member type of 'Container'}}
+}
+
+extension Container.Inner {
+  func extensionTest(obj: Container) {
+    obj.foo() // expected-error {{'foo' is inaccessible due to 'private' protection level}}
+    _ = obj.bar // expected-error {{'bar' is inaccessible due to 'private' protection level}}
+    obj.bar = 5 // expected-error {{'bar' is inaccessible due to 'private' protection level}}
+    obj.privateExtensionMethod() // expected-error {{'privateExtensionMethod' is inaccessible due to 'private' protection level}}
+
+    _ = PrivateInner() // FIXME expected-error {{use of unresolved identifier 'PrivateInner'}}
+    _ = Container.PrivateInner() // expected-error {{'PrivateInner' is inaccessible due to 'private' protection level}}
+  }
+
+  // FIXME: Why do these errors happen twice?
+  var inner: PrivateInner? { return nil } // FIXME expected-error 2 {{use of undeclared type 'PrivateInner'}}
+  var innerQualified: Container.PrivateInner? { return nil } // FIXME expected-error 2 {{'PrivateInner' is not a member type of 'Container'}}
+}
+
+class Sub : Container {
+  func subTest() {
+    foo() // FIXME expected-error {{use of unresolved identifier 'foo'}}
+    self.foo() // expected-error {{'foo' is inaccessible due to 'private' protection level}}
+
+    _ = bar // FIXME expected-error {{use of unresolved identifier 'bar'}}
+    bar = 5 // FIXME expected-error {{use of unresolved identifier 'bar'}}
+    _ = self.bar // expected-error {{'bar' is inaccessible due to 'private' protection level}}
+    self.bar = 5 // expected-error {{'bar' is inaccessible due to 'private' protection level}}
+
+    privateExtensionMethod() // FIXME expected-error {{use of unresolved identifier 'privateExtensionMethod'}}
+    self.privateExtensionMethod() // expected-error {{'privateExtensionMethod' is inaccessible due to 'private' protection level}}
+
+    _ = PrivateInner() // FIXME expected-error {{use of unresolved identifier 'PrivateInner'}}
+    _ = Container.PrivateInner() // expected-error {{'PrivateInner' is inaccessible due to 'private' protection level}}
+  }
+
+  var subInner: PrivateInner? // FIXME expected-error {{use of undeclared type 'PrivateInner'}}
+  var subInnerQualified: Container.PrivateInner? // FIXME expected-error {{'PrivateInner' is not a member type of 'Container'}}
+}
diff --git a/validation-test/compiler_crashers_2_fixed/0016-rdar21437203.swift b/validation-test/compiler_crashers_2_fixed/0016-rdar21437203.swift
index f70ccaa63c444..f80d42b6f2f8f 100644
--- a/validation-test/compiler_crashers_2_fixed/0016-rdar21437203.swift
+++ b/validation-test/compiler_crashers_2_fixed/0016-rdar21437203.swift
@@ -5,7 +5,7 @@ struct Curds {
 }
 
 private class Butter {
-    private func churn<T>(block: () throws -> T) throws -> T {
+    fileprivate func churn<T>(block: () throws -> T) throws -> T {
       return try block()
     }
 }