diff --git a/CMakeLists.txt b/CMakeLists.txt
index c0306ccb76c19..3a4bb3a033637 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -108,7 +108,7 @@ set_property(CACHE SWIFT_ANALYZE_CODE_COVERAGE PROPERTY
 # SWIFT_VERSION is deliberately /not/ cached so that an existing build directory
 # can be reused when a new version of Swift comes out (assuming the user hasn't
 # manually set it as part of their own CMake configuration).
-set(SWIFT_VERSION "4.0")
+set(SWIFT_VERSION "4.1")
 
 set(SWIFT_VENDOR "" CACHE STRING
     "The vendor name of the Swift compiler")
diff --git a/docs/Testing.md b/docs/Testing.md
index 15af92e7a602b..b417eb4124b7d 100644
--- a/docs/Testing.md
+++ b/docs/Testing.md
@@ -419,7 +419,7 @@ FIXME: full list.
 
 * ``optimized_stdlib_<CPUNAME>``: an optimized stdlib plus cpu configuration.
 
-* ``SWIFT_VERSION=<MAJOR>``: restricts a test to Swift 3 or Swift 4. If you
+* ``SWIFT_VERSION=<MAJOR>``: restricts a test to Swift 3, Swift 4, Swift 5. If you
   need to use this, make sure to add a test for the other version as well
   unless you are specifically testing ``-swift-version``-related functionality.
 
diff --git a/include/swift/Basic/Version.h b/include/swift/Basic/Version.h
index 8bcaf02083625..1519536896e81 100644
--- a/include/swift/Basic/Version.h
+++ b/include/swift/Basic/Version.h
@@ -137,8 +137,8 @@ class Version {
 
   // Whitelist of backward-compatibility versions that we permit passing as
   // -swift-version <vers>
-  static std::array<StringRef, 2> getValidEffectiveVersions() {
-    return {{"3", "4"}};
+  static std::array<StringRef, 3> getValidEffectiveVersions() {
+    return {{"3", "4", "5"}};
   };
 };
 
diff --git a/lib/Basic/Version.cpp b/lib/Basic/Version.cpp
index f99ce28659d70..e363a303a81b3 100644
--- a/lib/Basic/Version.cpp
+++ b/lib/Basic/Version.cpp
@@ -319,14 +319,16 @@ Optional<Version> Version::getEffectiveLanguageVersion() const {
   switch (Components[0]) {
   case 3:
 #ifdef SWIFT_VERSION_PATCHLEVEL
-    return Version{3, 2, SWIFT_VERSION_PATCHLEVEL};
+    return Version{3, 3, SWIFT_VERSION_PATCHLEVEL};
 #else
-    return Version{3, 2};
+    return Version{3, 3};
 #endif
   case 4:
     static_assert(SWIFT_VERSION_MAJOR == 4,
                   "getCurrentLanguageVersion is no longer correct here");
     return Version::getCurrentLanguageVersion();
+  case 5:
+    return Version{5, 0};
   default:
     return None;
   }
diff --git a/lib/ClangImporter/ImportName.cpp b/lib/ClangImporter/ImportName.cpp
index 97a8dcfdf815a..b7a4adb14c186 100644
--- a/lib/ClangImporter/ImportName.cpp
+++ b/lib/ClangImporter/ImportName.cpp
@@ -65,7 +65,9 @@ importer::nameVersionFromOptions(const LangOptions &langOpts) {
     return ImportNameVersion::Swift2;
   case 3:
     return ImportNameVersion::Swift3;
+  // Fixme: Figure out the importing story for 5 instead of falling back to 4.
   case 4:
+  case 5:
     return ImportNameVersion::Swift4;
   }
 }
diff --git a/test/Driver/swift-version-default.swift b/test/Driver/swift-version-default.swift
index 4db2b19129c50..32fe7f663ec8b 100644
--- a/test/Driver/swift-version-default.swift
+++ b/test/Driver/swift-version-default.swift
@@ -24,9 +24,9 @@ htn
 #endif
 
 #if swift(>=4.1)
-aoeu
+aoeu // expected-error {{use of unresolved identifier}}
 #else
-htn // expected-error {{use of unresolved identifier}}
+htn 
 #endif
 
 #if swift(>=5)
diff --git a/test/Driver/swift-version.swift b/test/Driver/swift-version.swift
index 129a2beb309d7..31ed2b48a7518 100644
--- a/test/Driver/swift-version.swift
+++ b/test/Driver/swift-version.swift
@@ -7,21 +7,25 @@
 // RUN: not %target-swiftc_driver -swift-version 3.0 %s 2>&1 | %FileCheck --check-prefix FIXIT_3 %s
 // RUN: not %target-swiftc_driver -swift-version 3.3 %s 2>&1 | %FileCheck --check-prefix FIXIT_3 %s
 // RUN: not %target-swiftc_driver -swift-version 4.3 %s 2>&1 | %FileCheck --check-prefix FIXIT_4 %s
+// RUN: not %target-swiftc_driver -swift-version 5.1 %s 2>&1 | %FileCheck --check-prefix FIXIT_5 %s
 
 // RUN: not %target-swiftc_driver -swift-version 3 -typecheck %s 2>&1 | %FileCheck --check-prefix ERROR_3 %s
 // RUN: not %target-swiftc_driver -swift-version 4 -typecheck %s 2>&1 | %FileCheck --check-prefix ERROR_4 %s
+// RUN: not %target-swiftc_driver -swift-version 5 -typecheck %s 2>&1 | %FileCheck --check-prefix ERROR_5 %s
 
 // BAD: invalid value
-// BAD: note: valid arguments to '-swift-version' are '3', '4'
+// BAD: note: valid arguments to '-swift-version' are '3', '4', '5'
 
 // FIXIT_3: use major version, as in '-swift-version 3'
 // FIXIT_4: use major version, as in '-swift-version 4'
+// FIXIT_5: use major version, as in '-swift-version 5'
 
 
 #if swift(>=3)
 asdf
 // ERROR_3: [[@LINE-1]]:1: error: {{use of unresolved identifier}}
 // ERROR_4: [[@LINE-2]]:1: error: {{use of unresolved identifier}}
+// ERROR_5: [[@LINE-3]]:1: error: {{use of unresolved identifier}}
 #else
 jkl
 #endif
@@ -30,26 +34,31 @@ jkl
 asdf
 // ERROR_3: [[@LINE-1]]:1: error: {{use of unresolved identifier}}
 // ERROR_4: [[@LINE-2]]:1: error: {{use of unresolved identifier}}
+// ERROR_5: [[@LINE-3]]:1: error: {{use of unresolved identifier}}
 #else
 jkl
 #endif
 
 #if swift(>=4)
-asdf // ERROR_4: [[@LINE]]:1: error: {{use of unresolved identifier}}
+asdf 
+// ERROR_4: [[@LINE-1]]:1: error: {{use of unresolved identifier}}
+// ERROR_5: [[@LINE-2]]:1: error: {{use of unresolved identifier}}
 #else
 jkl // ERROR_3: [[@LINE]]:1: error: {{use of unresolved identifier}}
 #endif
 
 #if swift(>=4.1)
 asdf
+// ERROR_4: [[@LINE-1]]:1: error: {{use of unresolved identifier}}
+// ERROR_5: [[@LINE-2]]:1: error: {{use of unresolved identifier}}
 #else
 jkl
 // ERROR_3: [[@LINE-1]]:1: error: {{use of unresolved identifier}}
-// ERROR_4: [[@LINE-2]]:1: error: {{use of unresolved identifier}}
 #endif
 
 #if swift(>=5)
 asdf
+// ERROR_5: [[@LINE-1]]:1: error: {{use of unresolved identifier}}
 #else
 jkl
 // ERROR_3: [[@LINE-1]]:1: error: {{use of unresolved identifier}}
diff --git a/test/Parse/ConditionalCompilation/language_version_explicit.swift b/test/Parse/ConditionalCompilation/language_version_explicit.swift
index 81e0eaad35a27..5baef8ad9f3d4 100644
--- a/test/Parse/ConditionalCompilation/language_version_explicit.swift
+++ b/test/Parse/ConditionalCompilation/language_version_explicit.swift
@@ -22,9 +22,9 @@
 #endif
 
 #if swift(>=4.0.1)
+  let z = 1
+#else
   // This shouldn't emit any diagnostics.
   asdf asdf asdf asdf
-#else
-  let z = 1
 #endif
 
diff --git a/test/Serialization/Recovery/crash-recovery.swift b/test/Serialization/Recovery/crash-recovery.swift
index 733375ad471fa..aecd4d26e3b3a 100644
--- a/test/Serialization/Recovery/crash-recovery.swift
+++ b/test/Serialization/Recovery/crash-recovery.swift
@@ -14,7 +14,7 @@ public class Sub: Base {
 
 // CHECK-CRASH: error: fatal error encountered while reading from module 'Lib'; please file a bug report with your project and the crash log
 // CHECK-CRASH-3-NOT: note
-// CHECK-CRASH-4: note: compiling as Swift 4.0, with 'Lib' built as Swift 3.2
+// CHECK-CRASH-4: note: compiling as Swift 4.1, with 'Lib' built as Swift 3.3
 // CHECK-CRASH-LABEL: *** DESERIALIZATION FAILURE (please include this section in any bug report) ***
 // CHECK-CRASH: could not find 'disappearingMethod()' in parent class
 // CHECK-CRASH: While loading members for 'Sub' in module 'Lib'
diff --git a/test/Serialization/Recovery/types-4-to-3.swift b/test/Serialization/Recovery/types-4-to-3.swift
index ca540c1b3812e..380c3d097977a 100644
--- a/test/Serialization/Recovery/types-4-to-3.swift
+++ b/test/Serialization/Recovery/types-4-to-3.swift
@@ -16,8 +16,8 @@ import Lib
 func requiresConformance(_: B_RequiresConformance<B_ConformsToProto>) {}
 func requiresConformance(_: B_RequiresConformance<C_RelyOnConformanceImpl.Assoc>) {}
 
-class Sub: Base {} // expected-error {{cannot inherit from class 'Base' (compiled with Swift 4.0) because it has overridable members that could not be loaded in Swift 3.2}}
-class Impl: Proto {} // expected-error {{type 'Impl' cannot conform to protocol 'Proto' (compiled with Swift 4.0) because it has requirements that could not be loaded in Swift 3.2}}
+class Sub: Base {} // expected-error {{cannot inherit from class 'Base' (compiled with Swift 4.1) because it has overridable members that could not be loaded in Swift 3.3}}
+class Impl: Proto {} // expected-error {{type 'Impl' cannot conform to protocol 'Proto' (compiled with Swift 4.1) because it has requirements that could not be loaded in Swift 3.3}}
 
 #else // TEST
 
diff --git a/utils/build-script b/utils/build-script
index 515728526e816..be142150a7ebd 100755
--- a/utils/build-script
+++ b/utils/build-script
@@ -2072,7 +2072,7 @@ iterations with -O",
         "--swift-user-visible-version",
         help="User-visible version of the embedded Swift compiler",
         type=arguments.type.swift_compiler_version,
-        default="4.0",
+        default="4.1",
         metavar="MAJOR.MINOR")
 
     parser.add_argument(