diff --git a/cmake/modules/SwiftSource.cmake b/cmake/modules/SwiftSource.cmake
index a7fa2cbe82124..3a227bea552a5 100644
--- a/cmake/modules/SwiftSource.cmake
+++ b/cmake/modules/SwiftSource.cmake
@@ -54,6 +54,9 @@ function(handle_swift_sources
   if (NOT SWIFTSOURCES_IS_MAIN)
     list(APPEND swift_compile_flags "-module-link-name" "${name}")
   endif()
+  if("${SWIFTSOURCES_SDK}" STREQUAL "CYGWIN")
+    list(APPEND swift_compile_flags -DCYGWIN)
+  endif()
 
   if(swift_sources)
     # Special-case hack to create Swift.o for the core standard library.
diff --git a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.c b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.c
index 5ad3fbe6b28a6..b64609ca1ad3a 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.c
+++ b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.c
@@ -10,8 +10,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-// Spawn is not available on Android.
-#if !defined(__ANDROID__)
+// posix_spawn is not available on Android or Windows (MSVC).
+#if !defined(__ANDROID__) && (!defined(_WIN32) || defined(__CYGWIN__))
 
 #include "swift/Runtime/Config.h"
 
@@ -65,5 +65,5 @@ char ***swift_SwiftPrivateLibcExtras_NSGetEnviron(void) {
   return _NSGetEnviron();
 }
 #endif // defined(__APPLE__)
-#endif // defined(__ANDROID__)
+#endif // !defined(__ANDROID__) && (!defined(_WIN32) || defined(__CGYWIN__))
 
diff --git a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
index 387734f00b87d..3ac4f636602eb 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
+++ b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
@@ -18,7 +18,8 @@ import Glibc
 #endif
 
 
-// posix_spawn is not available on Android.
+#if !os(Windows) || CYGWIN
+// posix_spawn is not available on Android or Windows.
 #if !os(Android)
 // swift_posix_spawn isn't available in the public watchOS SDK, we sneak by the
 // unavailable attribute declaration here of the APIs that we need.
@@ -293,3 +294,5 @@ internal func _getEnviron() -> UnsafeMutablePointer<UnsafeMutablePointer<CChar>?
   return __environ
 #endif
 }
+#endif
+
diff --git a/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift b/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
index 1b722422297b1..353a8f5d7f0a6 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
+++ b/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
@@ -17,6 +17,7 @@ import Darwin
 import Glibc
 #endif
 
+#if !os(Windows) || CYGWIN
 public func _stdlib_mkstemps(_ template: inout String, _ suffixlen: CInt) -> CInt {
 #if os(Android)
   preconditionFailure("mkstemps doesn't work on Android")
@@ -32,6 +33,7 @@ public func _stdlib_mkstemps(_ template: inout String, _ suffixlen: CInt) -> CIn
   return fd
 #endif
 }
+#endif
 
 public var _stdlib_FD_SETSIZE: CInt {
   return 1024
@@ -81,6 +83,7 @@ public struct _stdlib_fd_set {
   }
 }
 
+#if !os(Windows) || CYGWIN
 public func _stdlib_select(
   _ readfds: inout _stdlib_fd_set, _ writefds: inout _stdlib_fd_set,
   _ errorfds: inout _stdlib_fd_set, _ timeout: UnsafeMutablePointer<timeval>?
@@ -104,6 +107,7 @@ public func _stdlib_select(
     }
   }
 }
+#endif
 
 //
 // Functions missing in `Darwin` module.
diff --git a/stdlib/public/Platform/CMakeLists.txt b/stdlib/public/Platform/CMakeLists.txt
index 69db8da324d55..fb03347b044c0 100644
--- a/stdlib/public/Platform/CMakeLists.txt
+++ b/stdlib/public/Platform/CMakeLists.txt
@@ -22,6 +22,11 @@ add_swift_library(swiftGlibc ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_SDK_OVERLAY
     TARGET_SDKS ANDROID CYGWIN FREEBSD LINUX
     DEPENDS glibc_modulemap)
 
+add_swift_library(swiftMSVCRT ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_SDK_OVERLAY
+    msvcrt.swift
+    ${swift_platform_sources}
+    TARGET_SDKS WINDOWS)
+
 set(glibc_modulemap_target_list)
 foreach(sdk ${SWIFT_SDKS})
   if("${sdk}" STREQUAL "LINUX" OR
diff --git a/stdlib/public/Platform/Misc.c b/stdlib/public/Platform/Misc.c
index 7125026fbf99e..6473a63b00d80 100644
--- a/stdlib/public/Platform/Misc.c
+++ b/stdlib/public/Platform/Misc.c
@@ -11,18 +11,33 @@
 //===----------------------------------------------------------------------===//
 
 #include <fcntl.h>
+#if !defined(_WIN32) || defined(__CYGWIN__)
 #include <semaphore.h>
+#endif
+#if defined(_WIN32) && !defined(__CYGWIN__)
+#include <io.h>
+#endif
 #include <sys/types.h>
 #include <sys/stat.h>
+#if !defined(_WIN32) || defined(__CYGWIN__)
 #include <sys/ioctl.h>
+#endif
 
 #include "swift/Runtime/Config.h"
 
+#if !defined(_WIN32) || defined(__CYGWIN__)
 SWIFT_CC(swift)
 extern int _swift_Platform_open(const char *path, int oflag, mode_t mode) {
   return open(path, oflag, mode);
 }
+#else
+SWIFT_CC(swift)
+extern int _swift_Platform_open(const char *path, int oflag, int mode) {
+  return _open(path, oflag, mode);
+}
+#endif
 
+#if !defined(_WIN32) || defined(__CYGWIN__)
 SWIFT_CC(swift)
 extern int _swift_Platform_openat(int fd, const char *path, int oflag,
                                   mode_t mode) {
@@ -61,7 +76,8 @@ extern int
 _swift_Platform_ioctlPtr(int fd, unsigned long int request, void* ptr) {
   return ioctl(fd, request, ptr);
 }
- 
+#endif
+
 #if defined(__APPLE__)
 #define _REENTRANT
 #include <math.h>
diff --git a/stdlib/public/Platform/Platform.swift b/stdlib/public/Platform/Platform.swift
index bcd62b845d2e7..038a9fb35d2bd 100644
--- a/stdlib/public/Platform/Platform.swift
+++ b/stdlib/public/Platform/Platform.swift
@@ -98,9 +98,14 @@ public var errno : Int32 {
   get {
 #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS) || os(FreeBSD) || os(PS4)
     return __error().pointee
-// FIXME: os(Windows) should be replaced, such as triple(Cygwin)
-#elseif os(Android) || os(Windows)
+#elseif os(Android)
     return __errno().pointee
+#elseif os(Windows)
+#if CYGWIN
+    return __errno().pointee
+#else
+    return _errno().pointee
+#endif
 #else
     return __errno_location().pointee
 #endif
@@ -108,8 +113,14 @@ public var errno : Int32 {
   set(val) {
 #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS) || os(FreeBSD) || os(PS4)
     return __error().pointee = val
-#elseif os(Android) || os(Windows)
+#elseif os(Android)
+    return __errno().pointee = val
+#elseif os(Windows)
+#if CYGWIN
     return __errno().pointee = val
+#else
+    return _errno().pointee = val
+#endif
 #else
     return __errno_location().pointee = val
 #endif
@@ -165,13 +176,23 @@ public func snprintf(ptr: UnsafeMutablePointer<Int8>, _ len: Int, _ format: Unsa
 // fcntl.h
 //===----------------------------------------------------------------------===//
 
+#if !os(Windows) || CYGWIN
 @_silgen_name("_swift_Platform_open")
 func _swift_Platform_open(
   _ path: UnsafePointer<CChar>,
   _ oflag: Int32,
   _ mode: mode_t
 ) -> Int32
+#else
+@_silgen_name("_swift_Platform_open")
+func _swift_Platform_open(
+  _ path: UnsafePointer<CChar>,
+  _ oflag: Int32,
+  _ mode: Int32
+) -> Int32
+#endif
 
+#if !os(Windows) || CYGWIN
 @_silgen_name("_swift_Platform_openat")
 func _swift_Platform_openat(
   _ fd: Int32,
@@ -179,6 +200,7 @@ func _swift_Platform_openat(
   _ oflag: Int32,
   _ mode: mode_t
 ) -> Int32
+#endif
 
 public func open(
   _ path: UnsafePointer<CChar>,
@@ -187,6 +209,7 @@ public func open(
   return _swift_Platform_open(path, oflag, 0)
 }
 
+#if !os(Windows) || CYGWIN
 public func open(
   _ path: UnsafePointer<CChar>,
   _ oflag: Int32,
@@ -211,7 +234,17 @@ public func openat(
 ) -> Int32 {
   return _swift_Platform_openat(fd, path, oflag, mode)
 }
+#else
+public func open(
+  _ path: UnsafePointer<CChar>,
+  _ oflag: Int32,
+  _ mode: Int32
+) -> Int32 {
+  return _swift_Platform_open(path, oflag, mode)
+}
+#endif
 
+#if !os(Windows) || CYGWIN
 @_silgen_name("_swift_Platform_fcntl")
 internal func _swift_Platform_fcntl(
   _ fd: Int32,
@@ -248,7 +281,9 @@ public func fcntl(
 ) -> Int32 {
   return _swift_Platform_fcntlPtr(fd, cmd, ptr)
 }
+#endif
 
+#if !os(Windows) || CYGWIN
 public var S_IFMT: mode_t   { return mode_t(0o170000) }
 public var S_IFIFO: mode_t  { return mode_t(0o010000) }
 public var S_IFCHR: mode_t  { return mode_t(0o020000) }
@@ -286,11 +321,24 @@ public var S_IREAD: mode_t  { return S_IRUSR }
 public var S_IWRITE: mode_t { return S_IWUSR }
 public var S_IEXEC: mode_t  { return S_IXUSR }
 #endif
+#else
+public var S_IFMT: Int32 { return Int32(0xf000) }
+
+public var S_IFREG: Int32 { return Int32(0x8000) }
+public var S_IFDIR: Int32 { return Int32(0x4000) }
+public var S_IFCHR: Int32 { return Int32(0x2000) }
+public var S_IFIFO: Int32 { return Int32(0x1000) }
+
+public var S_IREAD: Int32  { return Int32(0x0100) }
+public var S_IWRITE: Int32 { return Int32(0x0080) }
+public var S_IEXEC: Int32  { return Int32(0x0040) }
+#endif
 
 //===----------------------------------------------------------------------===//
 // ioctl.h
 //===----------------------------------------------------------------------===//
 
+#if !os(Windows) || CYGWIN
 @_silgen_name("_swift_Platform_ioctl")
 internal func _swift_Platform_ioctl(
   _ fd: CInt,
@@ -327,8 +375,8 @@ public func ioctl(
 ) -> CInt {
   return _swift_Platform_ioctl(fd, request, 0)
 }
- 
- 
+#endif
+
 //===----------------------------------------------------------------------===//
 // unistd.h
 //===----------------------------------------------------------------------===//
@@ -354,14 +402,22 @@ public var SIG_DFL: sig_t? { return nil }
 public var SIG_IGN: sig_t { return unsafeBitCast(1, to: sig_t.self) }
 public var SIG_ERR: sig_t { return unsafeBitCast(-1, to: sig_t.self) }
 public var SIG_HOLD: sig_t { return unsafeBitCast(5, to: sig_t.self) }
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Windows)
-#if os(Windows)
-// In Cygwin, the below SIG_* have the same value with Linux.
-// Verified with libstdc++6 v5.3.0 in Cygwin v2.4.1 64bit.
-public typealias sighandler_t = _sig_func_ptr
-#else
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
+public typealias sighandler_t = __sighandler_t
+
+public var SIG_DFL: sighandler_t? { return nil }
+public var SIG_IGN: sighandler_t {
+  return unsafeBitCast(1, to: sighandler_t.self)
+}
+public var SIG_ERR: sighandler_t {
+  return unsafeBitCast(-1, to: sighandler_t.self)
+}
+public var SIG_HOLD: sighandler_t {
+  return unsafeBitCast(2, to: sighandler_t.self)
+}
+#elseif os(Windows)
+#if CYGWIN
 public typealias sighandler_t = __sighandler_t
-#endif
 
 public var SIG_DFL: sighandler_t? { return nil }
 public var SIG_IGN: sighandler_t {
@@ -374,6 +430,15 @@ public var SIG_HOLD: sighandler_t {
   return unsafeBitCast(2, to: sighandler_t.self)
 }
 #else
+public var SIG_DFL: _crt_signal_t? { return nil }
+public var SIG_IGN: _crt_signal_t {
+  return unsafeBitCast(1, to: _crt_signal_t.self)
+}
+public var SIG_ERR: _crt_signal_t {
+  return unsafeBitCast(-1, to: _crt_signal_t.self)
+}
+#endif
+#else
 internal var _ignore = _UnsupportedPlatformError()
 #endif
 
@@ -381,14 +446,22 @@ internal var _ignore = _UnsupportedPlatformError()
 // semaphore.h
 //===----------------------------------------------------------------------===//
 
+#if !os(Windows) || CYGWIN
 /// The value returned by `sem_open()` in the case of failure.
 public var SEM_FAILED: UnsafeMutablePointer<sem_t>? {
 #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
   // The value is ABI.  Value verified to be correct for OS X, iOS, watchOS, tvOS.
   return UnsafeMutablePointer<sem_t>(bitPattern: -1)
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Windows)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
   // The value is ABI.  Value verified to be correct on Glibc.
   return UnsafeMutablePointer<sem_t>(bitPattern: 0)
+#elseif os(Windows)
+#if CYGWIN
+  // The value is ABI.  Value verified to be correct on Glibc.
+  return UnsafeMutablePointer<sem_t>(bitPattern: 0)
+#else
+  _UnsupportedPlatformError()
+#endif
 #else
   _UnsupportedPlatformError()
 #endif
@@ -423,6 +496,7 @@ public func sem_open(
 ) -> UnsafeMutablePointer<sem_t>? {
   return _swift_Platform_sem_open4(name, oflag, mode, value)
 }
+#endif
 
 //===----------------------------------------------------------------------===//
 // Misc.
diff --git a/stdlib/public/Platform/msvcrt.swift b/stdlib/public/Platform/msvcrt.swift
new file mode 100644
index 0000000000000..7bca53015a20e
--- /dev/null
+++ b/stdlib/public/Platform/msvcrt.swift
@@ -0,0 +1,15 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+@_exported import ucrt // Clang module
+@_exported import visualc // Clang module
+
diff --git a/stdlib/public/Platform/tgmath.swift.gyb b/stdlib/public/Platform/tgmath.swift.gyb
index a568cc8c36e5d..b434789f3c99c 100644
--- a/stdlib/public/Platform/tgmath.swift.gyb
+++ b/stdlib/public/Platform/tgmath.swift.gyb
@@ -171,11 +171,25 @@ public func fpclassify(_ value: ${T}) -> Int {
 %if T == 'Double':
 #if os(Linux)
   return Int(__fpclassify(CDouble(value)))
+#elseif os(Windows)
+#if CYGWIN
+  return Int(__fpclassify(CDouble(value)))
+#else
+  return Int(_dclass(CDouble(value)))
+#endif
 #else
   return Int(__fpclassifyd(CDouble(value)))
 #endif
 %else:
+#if os(Windows)
+#if CYGWIN
+  return Int(__fpclassify${f}(${CT}(value)))
+#else
+  return Int(_${f}dclass(${CT}(value)))
+#endif
+#else
   return Int(__fpclassify${f}(${CT}(value)))
+#endif
 %end
 }
 
@@ -247,7 +261,15 @@ public func scalbn(_ x: ${T}, _ n: Int) -> ${T} {
 
 % # This is AllFloatTypes not OverlayFloatTypes because of the tuple return.
 % for T, CT, f in AllFloatTypes():
-#if os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Windows)
+#if os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
+@_transparent
+public func lgamma(_ x: ${T}) -> (${T}, Int) {
+  var sign = Int32(0)
+  let value = lgamma${f}_r(${CT}(x), &sign)
+  return (${T}(value), Int(sign))
+}
+#elseif os(Windows)
+#if CYGWIN
 @_transparent
 public func lgamma(_ x: ${T}) -> (${T}, Int) {
   var sign = Int32(0)
@@ -255,6 +277,9 @@ public func lgamma(_ x: ${T}) -> (${T}, Int) {
   return (${T}(value), Int(sign))
 }
 #else
+// TODO(compnerd): implement
+#endif
+#else
 % # On Darwin platform,
 % # The real lgamma_r is not imported because it hides behind macro _REENTRANT.
 @_versioned
@@ -305,12 +330,28 @@ public func fma(_ x: ${T}, _ y: ${T}, _ z: ${T}) -> ${T} {
 % # These C functions only support double. The overlay fixes the Int parameter.
 @_transparent
 public func jn(_ n: Int, _ x: Double) -> Double {
+#if os(Windows)
+#if CYGWIN
+  return jn(Int32(n), x)
+#else
+  return _jn(Int32(n), x)
+#endif
+#else
   return jn(Int32(n), x)
+#endif
 }
 
 @_transparent
 public func yn(_ n: Int, _ x: Double) -> Double {
+#if os(Windows)
+#if CYGWIN
+  return yn(Int32(n), x)
+#else
+  return _yn(Int32(n), x)
+#endif
+#else
   return yn(Int32(n), x)
+#endif
 }
 
 % end
diff --git a/stdlib/public/Platform/ucrt.modulemap b/stdlib/public/Platform/ucrt.modulemap
new file mode 100644
index 0000000000000..bb082319be7fe
--- /dev/null
+++ b/stdlib/public/Platform/ucrt.modulemap
@@ -0,0 +1,102 @@
+//===--- ucrt.modulemap ---------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+module ucrt [system] {
+  module C {
+    module complex {
+      header "complex.h"
+      export *
+    }
+
+    module ctype {
+      header "ctype.h"
+      export *
+    }
+
+    module errno {
+      header "errno.h"
+      export *
+    }
+
+    module fenv {
+      header "fenv.h"
+      export *
+    }
+
+    module inttypes {
+      header "inttypes.h"
+      export *
+    }
+
+    module locale {
+      header "locale.h"
+      export *
+    }
+
+    module math {
+      header "math.h"
+      export *
+    }
+
+    module signal {
+      header "signal.h"
+      export *
+    }
+
+    module stddef {
+      header "stddef.h"
+      export *
+    }
+
+    module stdio {
+      header "stdio.h"
+      export *
+    }
+
+    module stdlib {
+      header "stdlib.h"
+      export *
+    }
+
+    module string {
+      header "string.h"
+      export *
+    }
+
+    module time {
+      header "time.h"
+      export *
+    }
+
+    module POSIX {
+      module sys {
+        export *
+
+        module stat {
+          header "sys/stat.h"
+          export *
+        }
+
+        module types {
+          header "sys/types.h"
+          export *
+        }
+
+        module utime {
+          header "sys/utime.h"
+          export *
+        }
+      }
+    }
+  }
+}
+
diff --git a/stdlib/public/Platform/visualc.modulemap b/stdlib/public/Platform/visualc.modulemap
new file mode 100644
index 0000000000000..15072003bb627
--- /dev/null
+++ b/stdlib/public/Platform/visualc.modulemap
@@ -0,0 +1,19 @@
+//===--- vc.modulemap -----------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+module visualc [system] {
+  module SAL {
+    header "sal.h"
+    export *
+  }
+}
+