diff --git a/docs/UsersManual.rst b/docs/UsersManual.rst
index a5d81488102bd081e0124f069b2ee9e2399fa6f8..9d8e978ed3695ce866208bbd69136bf98cf0d22c 100644
--- a/docs/UsersManual.rst
+++ b/docs/UsersManual.rst
@@ -73,7 +73,7 @@ Basic Usage
 Intro to how to use a C compiler for newbies.
 
 compile + link compile then link debug info enabling optimizations
-picking a language to use, defaults to C99 by default. Autosenses based
+picking a language to use, defaults to C11 by default. Autosenses based
 on extension. using a makefile
 
 Command Line Options
@@ -1474,9 +1474,12 @@ Differences between various standard modes
 ------------------------------------------
 
 clang supports the -std option, which changes what language mode clang
-uses. The supported modes for C are c89, gnu89, c94, c99, gnu99 and
-various aliases for those modes. If no -std option is specified, clang
-defaults to gnu99 mode.
+uses. The supported modes for C are c89, gnu89, c94, c99, gnu99, c11,
+gnu11, and various aliases for those modes. If no -std option is
+specified, clang defaults to gnu11 mode. Many C99 and C11 features are
+supported in earlier modes as a conforming extension, with a warning. Use
+``-pedantic-errors`` to request an error if a feature from a later standard
+revision is used in an earlier mode.
 
 Differences between all ``c*`` and ``gnu*`` modes:
 
@@ -1514,6 +1517,11 @@ Differences between ``*89`` and ``*99`` modes:
    in ``*89`` modes.
 -  Some warnings are different.
 
+Differences between ``*99`` and ``*11`` modes:
+
+-  Warnings for use of C11 features are disabled.
+-  ``__STDC_VERSION__`` is defined to ``201112L`` rather than ``199901L``.
+
 c94 mode is identical to c89 mode except that digraphs are enabled in
 c94 mode (FIXME: And ``__STDC_VERSION__`` should be defined!).
 
diff --git a/lib/Basic/Targets.cpp b/lib/Basic/Targets.cpp
index 2798601a51fb2f9da3386b9d62e89a23c6716ee9..957448a2d494e5d5ad57239d94886c597b87eaaf 100644
--- a/lib/Basic/Targets.cpp
+++ b/lib/Basic/Targets.cpp
@@ -531,8 +531,8 @@ protected:
     // Solaris headers require _XOPEN_SOURCE to be set to 600 for C99 and
     // newer, but to 500 for everything else.  feature_test.h has a check to
     // ensure that you are not using C99 with an old version of X/Open or C89
-    // with a new version.  
-    if (Opts.C99 || Opts.C11)
+    // with a new version.
+    if (Opts.C99)
       Builder.defineMacro("_XOPEN_SOURCE", "600");
     else
       Builder.defineMacro("_XOPEN_SOURCE", "500");
@@ -4584,7 +4584,7 @@ public:
     if (Opts.FastMath || Opts.FiniteMathOnly)
       Builder.defineMacro("__ARM_FP_FAST");
 
-    if ((Opts.C99 || Opts.C11) && !Opts.Freestanding)
+    if (Opts.C99 && !Opts.Freestanding)
       Builder.defineMacro("__ARM_FP_FENV_ROUNDING");
 
     Builder.defineMacro("__ARM_SIZEOF_WCHAR_T", Opts.ShortWChar ? "2" : "4");
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 19eecdbaedfb4f10b0fde6ebf76fb47f39e62b1f..373842d238173e8011fd64e55b2fde17af205259 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -1129,7 +1129,7 @@ void CompilerInvocation::setLangDefaults(LangOptions &Opts, InputKind IK,
     case IK_PreprocessedC:
     case IK_ObjC:
     case IK_PreprocessedObjC:
-      LangStd = LangStandard::lang_gnu99;
+      LangStd = LangStandard::lang_gnu11;
       break;
     case IK_CXX:
     case IK_PreprocessedCXX:
diff --git a/test/Lexer/has_extension.c b/test/Lexer/has_extension.c
index 3b08510aa4402184a7c7774e552f7a0fc7e3653f..2ab3d1df37a8193fd23ddb96f24c54c5cac6d425 100644
--- a/test/Lexer/has_extension.c
+++ b/test/Lexer/has_extension.c
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -E %s -o - | FileCheck --check-prefix=CHECK-PED-NONE %s
-// RUN: %clang_cc1 -pedantic-errors -E %s -o - | FileCheck --check-prefix=CHECK-PED-ERR %s
+// RUN: %clang_cc1 -std=c99 -E %s -o - | FileCheck --check-prefix=CHECK-PED-NONE %s
+// RUN: %clang_cc1 -std=c99 -pedantic-errors -E %s -o - | FileCheck --check-prefix=CHECK-PED-ERR %s
 
 // CHECK-PED-NONE: no_dummy_extension
 #if !__has_extension(dummy_extension)
diff --git a/test/Lexer/has_feature_c1x.c b/test/Lexer/has_feature_c1x.c
index e26e309c037101ecf7fcb4501bb34e721985285c..cba329cfb5501aaa2f73e9620afc325b33c1f200 100644
--- a/test/Lexer/has_feature_c1x.c
+++ b/test/Lexer/has_feature_c1x.c
@@ -1,5 +1,11 @@
-// RUN: %clang_cc1 -E -triple x86_64-linux-gnu -std=c1x %s -o - | FileCheck --check-prefix=CHECK-1X %s
-// RUN: %clang_cc1 -E %s -o - | FileCheck --check-prefix=CHECK-NO-1X %s
+// RUN: %clang_cc1 -E -triple x86_64-linux-gnu -std=c89 %s -o - | FileCheck --check-prefix=CHECK-NO-1X %s
+// RUN: %clang_cc1 -E -triple x86_64-linux-gnu -std=iso9899:199409 %s -o - | FileCheck --check-prefix=CHECK-NO-1X %s
+// RUN: %clang_cc1 -E -triple x86_64-linux-gnu -std=c99 %s -o - | FileCheck --check-prefix=CHECK-NO-1X %s
+// RUN: %clang_cc1 -E -triple x86_64-linux-gnu -std=c11 %s -o - | FileCheck --check-prefix=CHECK-1X %s
+//
+// RUN: %clang_cc1 -E -triple x86_64-linux-gnu -std=gnu89 %s -o - | FileCheck --check-prefix=CHECK-NO-1X %s
+// RUN: %clang_cc1 -E -triple x86_64-linux-gnu -std=gnu99 %s -o - | FileCheck --check-prefix=CHECK-NO-1X %s
+// RUN: %clang_cc1 -E -triple x86_64-linux-gnu -std=gnu11 %s -o - | FileCheck --check-prefix=CHECK-1X %s
 
 #if __has_feature(c_atomic)
 int has_atomic();
diff --git a/test/Parser/c11-noreturn.c b/test/Parser/c11-noreturn.c
index e61901dfb7918077c25ced82fb56a309cc0b7e39..6c01b5533acfcfccf29f579c0699561f273070a2 100644
--- a/test/Parser/c11-noreturn.c
+++ b/test/Parser/c11-noreturn.c
@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 -std=c11 -fsyntax-only -verify %s
-// RUN: not %clang_cc1 -pedantic -fsyntax-only %s 2>&1 | FileCheck -check-prefix=CHECK-EXT %s
+// RUN: not %clang_cc1 -std=c99 -pedantic -fsyntax-only %s 2>&1 | FileCheck -check-prefix=CHECK-EXT %s
 
 _Noreturn int f();
 int _Noreturn f(); // expected-note {{previous}}
diff --git a/test/Parser/c1x-alignas.c b/test/Parser/c1x-alignas.c
index 5b29df262d3ca82950643fe219a4b2f1a9318a64..ce8436c12fbb8c640bb3ad7db26fd2814fd283c3 100644
--- a/test/Parser/c1x-alignas.c
+++ b/test/Parser/c1x-alignas.c
@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 -std=c11 -fsyntax-only -verify %s
-// RUN: not %clang_cc1 -pedantic -fsyntax-only %s 2>&1 | FileCheck -check-prefix=CHECK-EXT %s
+// RUN: not %clang_cc1 -std=c99 -pedantic -fsyntax-only %s 2>&1 | FileCheck -check-prefix=CHECK-EXT %s
 
 _Alignas(4) char c1;
 unsigned _Alignas(long) char c2;
diff --git a/test/Preprocessor/init.c b/test/Preprocessor/init.c
index b45c2c944a16d6f8fc5ee88d88b0c3fefb644a3b..1afdbcaaa94f0e9e42f6be4e551419b0e7feac7f 100644
--- a/test/Preprocessor/init.c
+++ b/test/Preprocessor/init.c
@@ -80,7 +80,7 @@
 // COMMON:#define __ORDER_LITTLE_ENDIAN__ 1234
 // COMMON:#define __ORDER_PDP_ENDIAN__ 3412
 // COMMON:#define __STDC_HOSTED__ 1
-// COMMON:#define __STDC_VERSION__
+// COMMON:#define __STDC_VERSION__ 201112L
 // COMMON:#define __STDC__ 1
 // COMMON:#define __VERSION__
 // COMMON:#define __clang__ 1
@@ -2546,7 +2546,7 @@
 // MIPS32BE:#define __SIZE_TYPE__ unsigned int
 // MIPS32BE:#define __SIZE_WIDTH__ 32
 // MIPS32BE:#define __STDC_HOSTED__ 0
-// MIPS32BE:#define __STDC_VERSION__ 199901L
+// MIPS32BE:#define __STDC_VERSION__ 201112L
 // MIPS32BE:#define __STDC__ 1
 // MIPS32BE:#define __UINT16_C_SUFFIX__ {{$}}
 // MIPS32BE:#define __UINT16_MAX__ 65535
@@ -5458,7 +5458,7 @@
 // PPC-DARWIN:#define __SIZE_TYPE__ long unsigned int
 // PPC-DARWIN:#define __SIZE_WIDTH__ 32
 // PPC-DARWIN:#define __STDC_HOSTED__ 0
-// PPC-DARWIN:#define __STDC_VERSION__ 199901L
+// PPC-DARWIN:#define __STDC_VERSION__ 201112L
 // PPC-DARWIN:#define __STDC__ 1
 // PPC-DARWIN:#define __UINT16_C_SUFFIX__ {{$}}
 // PPC-DARWIN:#define __UINT16_MAX__ 65535
diff --git a/test/Preprocessor/line-directive.c b/test/Preprocessor/line-directive.c
index 0dd658f7f1a1333127b85d510f77c795ab75aef1..2ebe87e4680d09b6c6a021f00b66b525d19c8529 100644
--- a/test/Preprocessor/line-directive.c
+++ b/test/Preprocessor/line-directive.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -pedantic %s
+// RUN: %clang_cc1 -std=c99 -fsyntax-only -verify -pedantic %s
 // RUN: not %clang_cc1 -E %s 2>&1 | grep 'blonk.c:92:2: error: ABC'
 // RUN: not %clang_cc1 -E %s 2>&1 | grep 'blonk.c:93:2: error: DEF'
 
diff --git a/test/Sema/anonymous-struct-union-c11.c b/test/Sema/anonymous-struct-union-c11.c
index 229ee520575e11f7f3beab01e3aded11129c4ecb..712e29e5496a37cd6533f3d82543066c8cf34ec9 100644
--- a/test/Sema/anonymous-struct-union-c11.c
+++ b/test/Sema/anonymous-struct-union-c11.c
@@ -1,8 +1,8 @@
 // Check for warnings in non-C11 mode:
-// RUN: %clang_cc1 -fsyntax-only -verify -Wc11-extensions %s
+// RUN: %clang_cc1 -fsyntax-only -std=c99 -verify -Wc11-extensions %s
 
 // Expect no warnings in C11 mode:
-// RUN: %clang_cc1 -fsyntax-only -pedantic -Werror -std=c11 %s
+// RUN: %clang_cc1 -fsyntax-only -std=c11 -pedantic -Werror %s
 
 struct s {
   int a;
diff --git a/test/Sema/array-init.c b/test/Sema/array-init.c
index 4cc5e412c2d557d99033b5afb6a05be2cf23f008..7cb48153168fb046fd34e2dce550fe99e24bdeaf 100644
--- a/test/Sema/array-init.c
+++ b/test/Sema/array-init.c
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -pedantic -verify %s
-// RUN: %clang_cc1 -fsyntax-only -Wgnu -Wc11-extensions -verify %s
+// RUN: %clang_cc1 -std=gnu99 -fsyntax-only -pedantic -verify %s
+// RUN: %clang_cc1 -std=gnu99 -fsyntax-only -Wgnu -Wc11-extensions -verify %s
 // REQUIRES: LP64
 
 extern int foof() = 1; // expected-error{{illegal initializer (only variables can be initialized)}}
diff --git a/test/Sema/attr-deprecated.c b/test/Sema/attr-deprecated.c
index c9e3dd546c581f13597a0a1bc6da0f9ea7c96e79..2e3e722942a0f88a8d5907ad9c6103d586f4de24 100644
--- a/test/Sema/attr-deprecated.c
+++ b/test/Sema/attr-deprecated.c
@@ -121,6 +121,6 @@ struct test22 {
   __attribute((deprecated)) foo_dep e, f;
 };
 
-typedef int test23_ty __attribute((deprecated)); // expected-note {{previous definition is here}}
-typedef int test23_ty; // expected-note {{'test23_ty' has been explicitly marked deprecated here}} expected-warning {{redefinition of typedef 'test23_ty' is a C11 feature}}
+typedef int test23_ty __attribute((deprecated));
+typedef int test23_ty; // expected-note {{'test23_ty' has been explicitly marked deprecated here}}
 test23_ty test23_v; // expected-warning {{'test23_ty' is deprecated}}
diff --git a/test/Sema/types.c b/test/Sema/types.c
index 6a22b20b0ea45983d41590310a03c2b23d0b669d..1ed8dae4c805d1c2eea608e5280ddd308ec2739c 100644
--- a/test/Sema/types.c
+++ b/test/Sema/types.c
@@ -30,12 +30,12 @@ int c() {
   int __int128; // expected-error {{cannot combine with previous}} expected-warning {{does not declare anything}}
 }
 // __int128_t is __int128; __uint128_t is unsigned __int128.
-typedef __int128 check_int_128; // expected-note {{here}}
-typedef __int128_t check_int_128; // expected-note {{here}} expected-warning {{redefinition}}
+typedef __int128 check_int_128;
+typedef __int128_t check_int_128; // expected-note {{here}}
 typedef int check_int_128; // expected-error {{different types ('int' vs '__int128_t' (aka '__int128'))}}
 
-typedef unsigned __int128 check_uint_128; // expected-note {{here}}
-typedef __uint128_t check_uint_128; // expected-note {{here}} expected-warning {{redefinition}}
+typedef unsigned __int128 check_uint_128;
+typedef __uint128_t check_uint_128; // expected-note {{here}}
 typedef int check_uint_128; // expected-error {{different types ('int' vs '__uint128_t' (aka 'unsigned __int128'))}}
 
 // Array type merging should convert array size to whatever matches the target
diff --git a/www/compatibility.html b/www/compatibility.html
index 8bfaff191cb5b39f1120f72fa5c0ac8c0c507d5a..293be6f2203283b6124b0ad1b6a921b6e3f5d9f8 100644
--- a/www/compatibility.html
+++ b/www/compatibility.html
@@ -83,10 +83,10 @@
 <!-- ======================================================================= -->
 <h3 id="inline">C99 inline functions</h3>
 <!-- ======================================================================= -->
-<p>By default, Clang builds C code according to the C99 standard,
-which provides different semantics for the <code>inline</code> keyword
-than GCC's default behavior. For example, consider the following
-code:</p>
+<p>By default, Clang builds C code in GNU C11 mode, so it uses standard C99
+semantics for the <code>inline</code> keyword. These semantics are different
+from those in GNU C89 mode, which is the default mode in versions of GCC
+prior to 5.0. For example, consider the following code:</p>
 <pre>
 inline int add(int i, int j) { return i + j; }
 
@@ -110,10 +110,10 @@ Undefined symbols:
       _main in cc-y1jXIr.o
 </pre>
 
-<p>By contrast, GCC's default behavior follows the GNU89 dialect,
-which is the C89 standard plus a lot of extensions.  C89 doesn't have
-an <code>inline</code> keyword, but GCC recognizes it as an extension
-and just treats it as a hint to the optimizer.</p>
+<p>By contrast, GNU C89 mode (used by default in older versions of GCC) is the
+C89 standard plus a lot of extensions. C89 doesn't have an <code>inline</code>
+keyword, but GCC recognizes it as an extension and just treats it as a hint to
+the optimizer.</p>
 
 <p>There are several ways to fix this problem:</p>
 
@@ -130,12 +130,12 @@ and just treats it as a hint to the optimizer.</p>
   for a function to be inlined, nor does it guarantee that it will be.
   Some compilers ignore it completely.  Clang treats it as a mild
   suggestion from the programmer.</li>
-     
+
   <li>Provide an external (non-<code>inline</code>) definition
   of <code>add</code> somewhere else in your program.  The two
   definitions must be equivalent!</li>
 
-  <li>Compile with the GNU89 dialect by adding
+  <li>Compile in the GNU C89 dialect by adding
   <code>-std=gnu89</code> to the set of Clang options. This option is
   only recommended if the program source cannot be changed or if the
   program also relies on additional C89-specific behavior that cannot