diff --git a/docs/Modules.rst b/docs/Modules.rst
index cd430a2..6ac334d 100644
--- a/docs/Modules.rst
+++ b/docs/Modules.rst
@@ -148,6 +148,8 @@
 .. note::
 
   To actually see any benefits from modules, one first has to introduce module maps for the underlying C standard library and the libraries and headers on which it depends. The section `Modularizing a Platform`_ describes the steps one must take to write these module maps.
+  
+One can use module maps without modules to check the integrity of the use of header files. To do this, use the ``-fmodule-maps`` option instead of the ``-fmodules`` option.
 
 Compilation model
 -----------------
@@ -165,6 +167,9 @@
 ``-fcxx-modules``
   Enable the modules feature for C++ (EXPERIMENTAL and VERY BROKEN).
 
+``-fmodule-maps``
+  Enable interpretation of module maps (EXPERIMENTAL). This option is implied by ``-fmodules``.
+
 ``-fmodules-cache-path=<directory>``
   Specify the path to the modules cache. If not provided, Clang will select a system-appropriate default.
 
diff --git a/include/clang/Driver/Options.td b/include/clang/Driver/Options.td
index 28d9a5e..21f0417 100644
--- a/include/clang/Driver/Options.td
+++ b/include/clang/Driver/Options.td
@@ -557,6 +557,9 @@
 def fmodules : Flag <["-"], "fmodules">, Group<f_Group>,
   Flags<[DriverOption, CC1Option]>,
   HelpText<"Enable the 'modules' language feature">;
+def fmodule_maps : Flag <["-"], "fmodule-maps">, Group<f_Group>,
+  Flags<[DriverOption,CC1Option]>,
+  HelpText<"Read module maps to understand the structure of library headers">;
 def fmodules_ignore_macro : Joined<["-"], "fmodules-ignore-macro=">, Group<f_Group>, Flags<[CC1Option]>,
   HelpText<"Ignore the definition of the given macro when building and loading modules">;
 def fretain_comments_from_system_headers : Flag<["-"], "fretain-comments-from-system-headers">, Group<f_Group>, Flags<[CC1Option]>;
@@ -617,6 +620,8 @@
     Flags<[CC1Option]>, HelpText<"Disallow merging of constants">;
 def fno_modules : Flag <["-"], "fno-modules">, Group<f_Group>,
   Flags<[DriverOption]>;
+def fno_module_maps : Flag <["-"], "fno-module-maps">, Group<f_Group>,
+  Flags<[DriverOption]>;
 def fno_ms_extensions : Flag<["-"], "fno-ms-extensions">, Group<f_Group>;
 def fno_ms_compatibility : Flag<["-"], "fno-ms-compatibility">, Group<f_Group>;
 def fno_delayed_template_parsing : Flag<["-"], "fno-delayed-template-parsing">, Group<f_Group>;
diff --git a/include/clang/Lex/HeaderSearchOptions.h b/include/clang/Lex/HeaderSearchOptions.h
index afce5ba..830f48d 100644
--- a/include/clang/Lex/HeaderSearchOptions.h
+++ b/include/clang/Lex/HeaderSearchOptions.h
@@ -95,6 +95,9 @@
   /// Note: Only used for testing!
   unsigned DisableModuleHash : 1;
 
+  /// \brief Interpret module maps.  This option is implied by full modules.
+  unsigned ModuleMaps : 1;
+
   /// \brief The interval (in seconds) between pruning operations.
   ///
   /// This operation is expensive, because it requires Clang to walk through
@@ -134,7 +137,7 @@
 
 public:
   HeaderSearchOptions(StringRef _Sysroot = "/")
-    : Sysroot(_Sysroot), DisableModuleHash(0),
+    : Sysroot(_Sysroot), DisableModuleHash(0), ModuleMaps(0),
       ModuleCachePruneInterval(7*24*60*60),
       ModuleCachePruneAfter(31*24*60*60),
       UseBuiltinIncludes(true),
diff --git a/lib/Driver/Tools.cpp b/lib/Driver/Tools.cpp
index dfaffaa..254bf8b 100644
--- a/lib/Driver/Tools.cpp
+++ b/lib/Driver/Tools.cpp
@@ -3070,6 +3070,13 @@
     }
   }
 
+  // -fmodule-maps enables module map processing (off by default) for header
+  // checking.  It is implied by -fmodules.
+  if (Args.hasFlag(options::OPT_fmodule_maps, options::OPT_fno_module_maps,
+                   false)) {
+    CmdArgs.push_back("-fmodule-maps");
+  }
+
   // If a module path was provided, pass it along. Otherwise, use a temporary
   // directory.
   if (Arg *A = Args.getLastArg(options::OPT_fmodules_cache_path)) {
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index d362e33..27ddd84 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -872,6 +872,8 @@
   Opts.ResourceDir = Args.getLastArgValue(OPT_resource_dir);
   Opts.ModuleCachePath = Args.getLastArgValue(OPT_fmodules_cache_path);
   Opts.DisableModuleHash = Args.hasArg(OPT_fdisable_module_hash);
+  // -fmodules implies -fmodule-maps
+  Opts.ModuleMaps = Args.hasArg(OPT_fmodule_maps) || Args.hasArg(OPT_fmodules);
   Opts.ModuleCachePruneInterval =
       getLastArgIntValue(Args, OPT_fmodules_prune_interval, 7 * 24 * 60 * 60);
   Opts.ModuleCachePruneAfter =
diff --git a/lib/Lex/PPDirectives.cpp b/lib/Lex/PPDirectives.cpp
index c70019f..ef0cbd6 100644
--- a/lib/Lex/PPDirectives.cpp
+++ b/lib/Lex/PPDirectives.cpp
@@ -17,6 +17,7 @@
 #include "clang/Basic/SourceManager.h"
 #include "clang/Lex/CodeCompletionHandler.h"
 #include "clang/Lex/HeaderSearch.h"
+#include "clang/Lex/HeaderSearchOptions.h"
 #include "clang/Lex/LexDiagnostic.h"
 #include "clang/Lex/LiteralSupport.h"
 #include "clang/Lex/MacroInfo.h"
@@ -1421,7 +1422,7 @@
   const FileEntry *File = LookupFile(
       FilenameLoc, Filename, isAngled, LookupFrom, CurDir,
       Callbacks ? &SearchPath : NULL, Callbacks ? &RelativePath : NULL,
-      getLangOpts().Modules? &SuggestedModule : 0);
+      HeaderInfo.getHeaderSearchOpts().ModuleMaps ? &SuggestedModule : 0);
 
   if (Callbacks) {
     if (!File) {
@@ -1435,13 +1436,15 @@
           
           // Try the lookup again, skipping the cache.
           File = LookupFile(FilenameLoc, Filename, isAngled, LookupFrom, CurDir,
-                            0, 0, getLangOpts().Modules? &SuggestedModule : 0,
-                            /*SkipCache*/true);
+                            0, 0, HeaderInfo.getHeaderSearchOpts().ModuleMaps
+                                      ? &SuggestedModule
+                                      : 0,
+                            /*SkipCache*/ true);
         }
       }
     }
     
-    if (!SuggestedModule) {
+    if (!SuggestedModule || !getLangOpts().Modules) {
       // Notify the callback object that we've seen an inclusion directive.
       Callbacks->InclusionDirective(HashLoc, IncludeTok, Filename, isAngled,
                                     FilenameRange, File,
@@ -1456,10 +1459,10 @@
       // brackets, we can attempt a lookup as though it were a quoted path to
       // provide the user with a possible fixit.
       if (isAngled) {
-        File = LookupFile(FilenameLoc, Filename, false, LookupFrom, CurDir, 
-                          Callbacks ? &SearchPath : 0, 
-                          Callbacks ? &RelativePath : 0, 
-                          getLangOpts().Modules ? &SuggestedModule : 0);
+        File = LookupFile(
+            FilenameLoc, Filename, false, LookupFrom, CurDir,
+            Callbacks ? &SearchPath : 0, Callbacks ? &RelativePath : 0,
+            HeaderInfo.getHeaderSearchOpts().ModuleMaps ? &SuggestedModule : 0);
         if (File) {
           SourceRange Range(FilenameTok.getLocation(), CharEnd);
           Diag(FilenameTok, diag::err_pp_file_not_found_not_fatal) << 
@@ -1477,7 +1480,7 @@
 
   // If we are supposed to import a module rather than including the header,
   // do so now.
-  if (SuggestedModule) {
+  if (SuggestedModule && getLangOpts().Modules) {
     // Compute the module access path corresponding to this module.
     // FIXME: Should we have a second loadModule() overload to avoid this
     // extra lookup step?
diff --git a/lib/Lex/Preprocessor.cpp b/lib/Lex/Preprocessor.cpp
index 66f23f1..035f751 100644
--- a/lib/Lex/Preprocessor.cpp
+++ b/lib/Lex/Preprocessor.cpp
@@ -734,7 +734,7 @@
   }
 
   // If we have a non-empty module path, load the named module.
-  if (!ModuleImportPath.empty()) {
+  if (!ModuleImportPath.empty() && getLangOpts().Modules) {
     Module *Imported = TheModuleLoader.loadModule(ModuleImportLoc,
                                                   ModuleImportPath,
                                                   Module::MacrosVisible,
diff --git a/lib/Sema/SemaCodeComplete.cpp b/lib/Sema/SemaCodeComplete.cpp
index b46392b..bf57597 100644
--- a/lib/Sema/SemaCodeComplete.cpp
+++ b/lib/Sema/SemaCodeComplete.cpp
@@ -3188,7 +3188,7 @@
                                  ? CXAvailability_Available
                                   : CXAvailability_NotAvailable));
     }
-  } else {
+  } else if (getLangOpts().Modules) {
     // Load the named module.
     Module *Mod = PP.getModuleLoader().loadModule(ImportLoc, Path,
                                                   Module::AllVisible,