diff --git a/clang-tools-extra/modularize/Modularize.cpp b/clang-tools-extra/modularize/Modularize.cpp
index d68e855..45d825d 100644
--- a/clang-tools-extra/modularize/Modularize.cpp
+++ b/clang-tools-extra/modularize/Modularize.cpp
@@ -90,10 +90,12 @@
 #include <iterator>
 #include <string>
 #include <vector>
+#include "PreprocessorTracker.h"
 
 using namespace clang::tooling;
 using namespace clang;
 using namespace llvm;
+using namespace Modularize;
 
 // Option to specify a file name for a list of header files to check.
 cl::opt<std::string>
@@ -382,8 +384,14 @@
 
 class CollectEntitiesConsumer : public ASTConsumer {
 public:
-  CollectEntitiesConsumer(EntityMap &Entities, Preprocessor &PP)
-      : Entities(Entities), PP(PP) {}
+  CollectEntitiesConsumer(EntityMap &Entities,
+                          PreprocessorTracker &preprocessorTracker,
+                          Preprocessor &PP, StringRef InFile)
+      : Entities(Entities), PPTracker(preprocessorTracker), PP(PP) {
+    PPTracker.handlePreprocessorEntry(PP, InFile);
+  }
+
+  ~CollectEntitiesConsumer() { PPTracker.handlePreprocessorExit(); }
 
   virtual void HandleTranslationUnit(ASTContext &Ctx) {
     SourceManager &SM = Ctx.getSourceManager();
@@ -409,33 +417,41 @@
 
 private:
   EntityMap &Entities;
+  PreprocessorTracker &PPTracker;
   Preprocessor &PP;
 };
 
 class CollectEntitiesAction : public SyntaxOnlyAction {
 public:
-  CollectEntitiesAction(EntityMap &Entities) : Entities(Entities) {}
+  CollectEntitiesAction(EntityMap &Entities,
+                        PreprocessorTracker &preprocessorTracker)
+      : Entities(Entities), PPTracker(preprocessorTracker) {}
 
 protected:
   virtual clang::ASTConsumer *CreateASTConsumer(CompilerInstance &CI,
                                                 StringRef InFile) {
-    return new CollectEntitiesConsumer(Entities, CI.getPreprocessor());
+    return new CollectEntitiesConsumer(Entities, PPTracker,
+                                       CI.getPreprocessor(), InFile);
   }
 
 private:
   EntityMap &Entities;
+  PreprocessorTracker &PPTracker;
 };
 
 class ModularizeFrontendActionFactory : public FrontendActionFactory {
 public:
-  ModularizeFrontendActionFactory(EntityMap &Entities) : Entities(Entities) {}
+  ModularizeFrontendActionFactory(EntityMap &Entities,
+                                  PreprocessorTracker &preprocessorTracker)
+      : Entities(Entities), PPTracker(preprocessorTracker) {}
 
   virtual CollectEntitiesAction *create() {
-    return new CollectEntitiesAction(Entities);
+    return new CollectEntitiesAction(Entities, PPTracker);
   }
 
 private:
   EntityMap &Entities;
+  PreprocessorTracker &PPTracker;
 };
 
 int main(int argc, const char **argv) {
@@ -464,10 +480,14 @@
   Compilations.reset(
       new FixedCompilationDatabase(Twine(PathBuf), CC1Arguments));
 
+  // Create preprocessor tracker, to watch for macro and conditional problems.
+  OwningPtr<PreprocessorTracker> PPTracker(PreprocessorTracker::create());
+
   // Parse all of the headers, detecting duplicates.
   EntityMap Entities;
   ClangTool Tool(*Compilations, Headers);
-  int HadErrors = Tool.run(new ModularizeFrontendActionFactory(Entities));
+  int HadErrors =
+      Tool.run(new ModularizeFrontendActionFactory(Entities, *PPTracker));
 
   // Create a place to save duplicate entity locations, separate bins per kind.
   typedef SmallVector<Location, 8> LocationArray;
@@ -515,6 +535,16 @@
     }
   }
 
+  // Complain about macro instance in header files that differ based on how
+  // they are included.
+  if (PPTracker->reportInconsistentMacros(errs()))
+    HadErrors = 1;
+
+  // Complain about preprocessor conditional directives in header files that
+  // differ based on how they are included.
+  if (PPTracker->reportInconsistentConditionals(errs()))
+    HadErrors = 1;
+
   // Complain about any headers that have contents that differ based on how
   // they are included.
   // FIXME: Could we provide information about which preprocessor conditionals
@@ -530,7 +560,7 @@
 
     HadErrors = 1;
     errs() << "error: header '" << H->first->getName()
-           << "' has different contents depending on how it was included\n";
+           << "' has different contents depending on how it was included.\n";
     for (unsigned I = 0, N = H->second.size(); I != N; ++I) {
       errs() << "note: '" << H->second[I].Name << "' in "
              << H->second[I].Loc.File->getName() << " at "