diff --git a/include/swift/AST/Module.h b/include/swift/AST/Module.h
index 5c49c843fa3..9f471af6c74 100644
--- a/include/swift/AST/Module.h
+++ b/include/swift/AST/Module.h
@@ -27,6 +27,7 @@
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/StringMap.h"
+#include "llvm/ADT/TinyPtrVector.h"
 
 namespace clang {
   class Module;
@@ -361,19 +362,8 @@ public:
   template <typename T>
   using IdentifierMap = llvm::DenseMap<Identifier, T>;
 
-  /// A map of operator names to InfixOperatorDecls.
-  /// Populated during name binding; the mapping will be incomplete until name
-  /// binding is complete.
   IdentifierMap<InfixOperatorDecl*> InfixOperators;
-
-  /// A map of operator names to PostfixOperatorDecls.
-  /// Populated during name binding; the mapping will be incomplete until name
-  /// binding is complete.
   IdentifierMap<PostfixOperatorDecl*> PostfixOperators;
-
-  /// A map of operator names to PrefixOperatorDecls.
-  /// Populated during name binding; the mapping will be incomplete until name
-  /// binding is complete.
   IdentifierMap<PrefixOperatorDecl*> PrefixOperators;
 
   enum SourceKind {
@@ -399,8 +389,10 @@ public:
   };
 
   /// Defines what phases of parsing and semantic analysis are complete for a
-  /// source file.  This should only be used for assertions and verification
-  /// purposes.
+  /// source file.
+  ///
+  /// Only files that have been fully processed (i.e. type-checked) will be
+  /// forwarded on to IRGen.
   ASTStage_t ASTStage = Parsing;
 
   SourceFile(TranslationUnit &tu, SourceKind K, Optional<unsigned> ImportID,
@@ -497,14 +489,36 @@ class TranslationUnit : public Module {
   /// lookups.
   ExternalNameLookup *ExternalLookup = nullptr;
 
-public:
-  // FIXME: Make private or eliminate altogether.
-  SourceFile *MainSourceFile;
+  TinyPtrVector<SourceFile *> SourceFiles;
 
+public:
   TranslationUnit(Identifier Name, ASTContext &C)
     : Module(ModuleKind::TranslationUnit, Name, C) {
   }
 
+  ArrayRef<SourceFile *> getSourceFiles() {
+    return SourceFiles;
+  }
+  ArrayRef<const SourceFile *> getSourceFiles() const {
+    return { SourceFiles.begin(), SourceFiles.size() };
+  }
+
+  void addSourceFile(SourceFile &newFile) {
+    // Require Main and REPL files to be the first file added.
+    assert(SourceFiles.empty() ||
+           newFile.Kind == SourceFile::Library ||
+           newFile.Kind == SourceFile::SIL);
+    SourceFiles.push_back(&newFile);
+  }
+
+  /// Convenience accessor for clients that know what kind of file they're
+  /// dealing with.
+  SourceFile &getMainSourceFile(SourceFile::SourceKind expectedKind) const {
+    assert(!SourceFiles.empty() && "No files added yet");
+    assert(SourceFiles.front()->Kind == expectedKind);
+    return *SourceFiles.front();
+  }
+
   ExternalNameLookup *getExternalLookup() const { return ExternalLookup; }
   void setExternalLookup(ExternalNameLookup *R) {
     assert(!ExternalLookup && "Name resolver already set");
diff --git a/lib/AST/Module.cpp b/lib/AST/Module.cpp
index 37e2cff20cd..eed2cc931b9 100644
--- a/lib/AST/Module.cpp
+++ b/lib/AST/Module.cpp
@@ -276,8 +276,10 @@ void Module::lookupValue(AccessPathTy AccessPath, Identifier Name,
   }
 
   if (auto TU = dyn_cast<TranslationUnit>(this)) {
-    const SourceFile &SF = *TU->MainSourceFile;
-    return SF.getCache().lookupValue(AccessPath, Name, LookupKind, Result);
+    // FIXME: Access control.
+    for (const SourceFile *SF : TU->getSourceFiles())
+      SF->getCache().lookupValue(AccessPath, Name, LookupKind, Result);
+    return;
   }
 
   ModuleLoader &owner = cast<LoadedModule>(this)->getOwner();
@@ -294,8 +296,10 @@ void Module::lookupVisibleDecls(AccessPathTy AccessPath,
   }
 
   if (auto TU = dyn_cast<TranslationUnit>(this)) {
-    const SourceFile &SF = *TU->MainSourceFile;
-    return SF.getCache().lookupVisibleDecls(AccessPath, Consumer, LookupKind);
+    // FIXME: Access control.
+    for (const SourceFile *SF : TU->getSourceFiles())
+      SF->getCache().lookupVisibleDecls(AccessPath, Consumer, LookupKind);
+    return;
   }
 
   ModuleLoader &owner = cast<LoadedModule>(this)->getOwner();
@@ -310,8 +314,10 @@ void Module::lookupClassMembers(AccessPathTy accessPath,
   }
 
   if (auto TU = dyn_cast<TranslationUnit>(this)) {
-    const SourceFile &SF = *TU->MainSourceFile;
-    return SF.getCache().lookupClassMembers(accessPath, consumer, SF);
+    // FIXME: Access control.
+    for (const SourceFile *SF : TU->getSourceFiles())
+      SF->getCache().lookupClassMembers(accessPath, consumer, *SF);
+    return;
   }
 
   ModuleLoader &owner = cast<LoadedModule>(this)->getOwner();
@@ -327,8 +333,10 @@ void Module::lookupClassMember(AccessPathTy accessPath,
   }
   
   if (auto TU = dyn_cast<TranslationUnit>(this)) {
-    const SourceFile &SF = *TU->MainSourceFile;
-    return SF.getCache().lookupClassMember(accessPath, name, results, SF);
+    // FIXME: Access control.
+    for (const SourceFile *SF : TU->getSourceFiles())
+      SF->getCache().lookupClassMember(accessPath, name, results, *SF);
+    return;
   }
   
   ModuleLoader &owner = cast<LoadedModule>(this)->getOwner();
@@ -341,8 +349,9 @@ void Module::getTopLevelDecls(SmallVectorImpl<Decl*> &Results) {
   }
 
   if (auto TU = dyn_cast<TranslationUnit>(this)) {
-    Results.append(TU->MainSourceFile->Decls.begin(),
-                   TU->MainSourceFile->Decls.end());
+    // FIXME: Access control.
+    for (const SourceFile *SF : TU->getSourceFiles())
+      Results.append(SF->Decls.begin(), SF->Decls.end());
     return;
   }
 
@@ -807,8 +816,9 @@ void Module::getDisplayDecls(SmallVectorImpl<Decl*> &results) {
   }
   
   if (auto TU = dyn_cast<TranslationUnit>(this)) {
-    results.append(TU->MainSourceFile->Decls.begin(),
-                   TU->MainSourceFile->Decls.end());
+    // FIXME: Access control.
+    for (const SourceFile *SF : TU->getSourceFiles())
+      results.append(SF->Decls.begin(), SF->Decls.end());
     return;
   }
   
@@ -830,11 +840,13 @@ Optional<OP_DECL *> lookupOperatorDeclForName(Module *M,
 // Returns Nothing on error, Optional(nullptr) if no operator decl found, or
 // Optional(decl) if decl was found.
 template<typename OP_DECL>
-Optional<OP_DECL *> lookupOperatorDeclForName(SourceFile &SF,
+Optional<OP_DECL *> lookupOperatorDeclForName(const SourceFile &SF,
     SourceLoc Loc,
     Identifier Name,
     IdentifierMap<OP_DECL *> SourceFile::*OP_MAP)
 {
+  assert(SF.ASTStage >= SourceFile::NameBound);
+
   // Look for an operator declaration in the current module.
   auto found = (SF.*OP_MAP).find(Name);
   if (found != (SF.*OP_MAP).end())
@@ -852,11 +864,15 @@ Optional<OP_DECL *> lookupOperatorDeclForName(SourceFile &SF,
     if (OP_DECL *op = *maybeOp)
       importedOperators.insert(op);
   }
+
+  // FIXME: By caching our lookup, we are implicitly re-exporting the operator,
+  // and in a non-predictable way. Yuck.
+  auto &mutableOpMap = const_cast<IdentifierMap<OP_DECL *> &>(SF.*OP_MAP);
   
   // Return early if we didn't find anything.
   if (importedOperators.empty()) {
     // Cache the mapping so we don't need to troll imports next time.
-    (SF.*OP_MAP)[Name] = nullptr;
+    mutableOpMap[Name] = nullptr;
     return nullptr;
   }
 
@@ -875,7 +891,7 @@ Optional<OP_DECL *> lookupOperatorDeclForName(SourceFile &SF,
     }
   }
   // Cache the mapping so we don't need to troll imports next time.
-  (SF.*OP_MAP)[Name] = first;
+  mutableOpMap[Name] = first;
   return first;
 }
 
@@ -891,7 +907,21 @@ Optional<OP_DECL *> lookupOperatorDeclForName(Module *M,
   auto *TU = dyn_cast<TranslationUnit>(M);
   if (!TU)
     return nullptr;
-  return lookupOperatorDeclForName(*TU->MainSourceFile, Loc, Name, OP_MAP);
+
+  // FIXME: Access control.
+  OP_DECL *result = nullptr;
+  for (const SourceFile *SF : TU->getSourceFiles()) {
+    auto next = lookupOperatorDeclForName(*SF, Loc, Name, OP_MAP);
+    if (!next.hasValue())
+      return next;
+
+    // FIXME: Diagnose ambiguity.
+    if (*next && result)
+      return Nothing;
+    if (*next)
+      result = *next;
+  }
+  return result;
 }
 
 } // end anonymous namespace
@@ -925,9 +955,10 @@ Module::getImportedModules(SmallVectorImpl<ImportedModule> &modules,
     return;
 
   if (auto TU = dyn_cast<TranslationUnit>(this)) {
-    for (auto importPair : TU->MainSourceFile->getImports())
-      if (includePrivate || importPair.second)
-        modules.push_back(importPair.first);
+    for (auto SF : TU->getSourceFiles())
+      for (auto importPair : SF->getImports())
+        if (includePrivate || importPair.second)
+          modules.push_back(importPair.first);
     return;
   }
 
@@ -969,8 +1000,10 @@ StringRef Module::getModuleFilename() const {
     return StringRef();
 
   if (auto TU = dyn_cast<TranslationUnit>(this)) {
-    if (auto ID = TU->MainSourceFile->getImportBufferID())
-      return Ctx.SourceMgr->getMemoryBuffer(*ID)->getBufferIdentifier();
+    // FIXME: Figure out what the right intent is here.
+    if (TU->getSourceFiles().size() == 1)
+      if (auto ID = TU->getSourceFiles().front()->getImportBufferID())
+        return Ctx.SourceMgr->getMemoryBuffer(*ID)->getBufferIdentifier();
     return StringRef();
   }
 
@@ -1083,7 +1116,11 @@ SourceFile::getCachedVisibleDecls() const {
 }
 
 bool TranslationUnit::walk(ASTWalker &Walker) {
-  return MainSourceFile->walk(Walker);
+  llvm::SaveAndRestore<ASTWalker::ParentTy> SAR(Walker.Parent, this);
+  for (auto SF : getSourceFiles())
+    if (SF->walk(Walker))
+      return true;
+  return false;
 }
 
 static void performAutoImport(SourceFile &SF, bool hasBuiltinModuleAccess) {
diff --git a/lib/Frontend/Frontend.cpp b/lib/Frontend/Frontend.cpp
index a98d796ca66..3bd878f8a76 100644
--- a/lib/Frontend/Frontend.cpp
+++ b/lib/Frontend/Frontend.cpp
@@ -125,7 +125,7 @@ void swift::CompilerInstance::doIt() {
 
   auto *SingleInputFile =
     new (*Context) SourceFile(*TU, Kind, Invocation.getParseStdlib());
-  TU->MainSourceFile = SingleInputFile;
+  TU->addSourceFile(*SingleInputFile);
   if (Kind == SourceFile::REPL)
     return;
 
diff --git a/lib/IRGen/IRGen.cpp b/lib/IRGen/IRGen.cpp
index bc865edaf5a..2e4b2d149f7 100644
--- a/lib/IRGen/IRGen.cpp
+++ b/lib/IRGen/IRGen.cpp
@@ -169,7 +169,10 @@ static void performIRGeneration(Options &Opts, llvm::Module *Module,
     IGM.emitSourceFile(*SF, StartElem);
   } else {
     assert(StartElem == 0 && "no explicit source file provided");
-    IGM.emitSourceFile(*TU->MainSourceFile, 0);
+    for (auto *SF : TU->getSourceFiles()) {
+      if (SF->ASTStage == SourceFile::TypeChecked)
+        IGM.emitSourceFile(*SF, 0);
+    }
   }
   
   // Objective-C image information.
diff --git a/lib/SIL/SILPrinter.cpp b/lib/SIL/SILPrinter.cpp
index 1e34a97ac26..0e1332c5b48 100644
--- a/lib/SIL/SILPrinter.cpp
+++ b/lib/SIL/SILPrinter.cpp
@@ -1152,8 +1152,9 @@ void SILModule::print(llvm::raw_ostream &OS, bool Verbose,
     Options.VarInitializers = true;
     Options.SkipImplicit = true;
 
-    for (auto ID = TU->MainSourceFile->Decls.begin(),
-              ED = TU->MainSourceFile->Decls.end(); ID != ED; ++ID) {
+    // FIXME: What about the rest of the files in the TU?
+    for (auto ID = TU->getSourceFiles().front()->Decls.begin(),
+              ED = TU->getSourceFiles().front()->Decls.end(); ID != ED; ++ID) {
       const Decl *D = *ID;
       if ((isa<ValueDecl>(D) || isa<OperatorDecl>(D)) &&
           !emittedFunctions.count(D) &&
diff --git a/lib/SILGen/SILGen.cpp b/lib/SILGen/SILGen.cpp
index 24b5e23a62d..037f3cb6bda 100644
--- a/lib/SILGen/SILGen.cpp
+++ b/lib/SILGen/SILGen.cpp
@@ -583,10 +583,12 @@ std::unique_ptr<SILModule> SILModule::constructSIL(TranslationUnit *tu,
       sgm.visit(D);
   } else {
     assert(startElem == 0 && "no explicit source file");
-    // FIXME: Construct SIL for all source files, once there is more than one.
-    sf = tu->MainSourceFile;
-    for (Decl *D : llvm::makeArrayRef(sf->Decls).slice(startElem))
-      sgm.visit(D);
+    for (auto nextSF : tu->getSourceFiles()) {
+      if (nextSF->ASTStage != SourceFile::TypeChecked)
+        continue;
+      for (Decl *D : llvm::makeArrayRef(nextSF->Decls).slice(startElem))
+        sgm.visit(D);
+    }
   }
     
   // Emit external definitions used by this translation unit.
diff --git a/lib/Sema/SourceLoader.cpp b/lib/Sema/SourceLoader.cpp
index b2662176fdf..fd937d9f005 100644
--- a/lib/Sema/SourceLoader.cpp
+++ b/lib/Sema/SourceLoader.cpp
@@ -119,7 +119,7 @@ Module *SourceLoader::loadModule(SourceLoc importLoc,
 
   auto *importFile = new (Ctx) SourceFile(*importTU, SourceFile::Library,
                                           bufferID);
-  importTU->MainSourceFile = importFile;
+  importTU->addSourceFile(*importFile);
 
   bool done;
   PersistentParserState persistentState;
diff --git a/lib/Sema/TypeChecker.cpp b/lib/Sema/TypeChecker.cpp
index 4ffa0eef3fc..dd4c55cbd21 100644
--- a/lib/Sema/TypeChecker.cpp
+++ b/lib/Sema/TypeChecker.cpp
@@ -135,7 +135,7 @@ Module *TypeChecker::getStdlibModule() {
     // module.
     for (auto &entry : Context.LoadedModules) {
       if (auto TU = dyn_cast_or_null<TranslationUnit>(entry.getValue()))
-        if (!TU->MainSourceFile->getImportBufferID().hasValue())
+        if (!TU->getSourceFiles().front()->getImportBufferID().hasValue())
           StdlibModule = TU;
     }
     assert(StdlibModule && "no main module found");
@@ -481,14 +481,16 @@ void swift::performTypeChecking(SourceFile &SF, unsigned StartElem) {
     if (!importTU)
       return;
     // FIXME: Respect the access path?
-    for (auto D : importTU->MainSourceFile->Decls) {
-      if (auto ED = dyn_cast<ExtensionDecl>(D)) {
-        bindExtensionDecl(ED, TC);
-        if (mayConformToKnownProtocol(ED))
-          TC.validateDecl(ED->getExtendedType()->getAnyNominal());
-      } else if (auto nominal = dyn_cast<NominalTypeDecl>(D)) {
-        if (mayConformToKnownProtocol(nominal))
-          TC.validateDecl(nominal);
+    for (auto SF : importTU->getSourceFiles()) {
+      for (auto D : SF->Decls) {
+        if (auto ED = dyn_cast<ExtensionDecl>(D)) {
+          bindExtensionDecl(ED, TC);
+          if (mayConformToKnownProtocol(ED))
+            TC.validateDecl(ED->getExtendedType()->getAnyNominal());
+        } else if (auto nominal = dyn_cast<NominalTypeDecl>(D)) {
+          if (mayConformToKnownProtocol(nominal))
+            TC.validateDecl(nominal);
+        }
       }
     }
   });
diff --git a/lib/Serialization/Serialization.cpp b/lib/Serialization/Serialization.cpp
index 3b100ae2f0b..91c0103df37 100644
--- a/lib/Serialization/Serialization.cpp
+++ b/lib/Serialization/Serialization.cpp
@@ -390,13 +390,16 @@ void Serializer::writeInputFiles(const TranslationUnit *TU,
     SourceFile.emit(ScratchRecord, path);
   }
 
-  for (auto import : TU->MainSourceFile->getImports()) {
-    if (import.first.second == TU->Ctx.TheBuiltinModule)
-      continue;
+  for (auto SF : TU->getSourceFiles()) {
+    // FIXME: Do some uniquing.
+    for (auto import : SF->getImports()) {
+      if (import.first.second == TU->Ctx.TheBuiltinModule)
+        continue;
 
-    ImportPathBlob importPath;
-    flattenImportPath(import.first, importPath);
-    ImportedModule.emit(ScratchRecord, import.second, importPath);
+      ImportPathBlob importPath;
+      flattenImportPath(import.first, importPath);
+      ImportedModule.emit(ScratchRecord, import.second, importPath);
+    }
   }
 
   if (!moduleLinkName.empty()) {
@@ -2008,24 +2011,26 @@ void Serializer::writeTranslationUnit(const TranslationUnit *TU,
   writeSILFunctions(M);
 
   DeclTable topLevelDecls, extensionDecls, operatorDecls;
-  for (auto D : TU->MainSourceFile->Decls) {
-    if (isa<ImportDecl>(D))
-      continue;
-    else if (auto VD = dyn_cast<ValueDecl>(D)) {
-      if (VD->getName().empty())
+  for (auto SF : TU->getSourceFiles()) {
+    for (auto D : SF->Decls) {
+      if (isa<ImportDecl>(D))
         continue;
-      topLevelDecls[VD->getName()]
-        .push_back({ getKindForTable(D), addDeclRef(D) });
+      else if (auto VD = dyn_cast<ValueDecl>(D)) {
+        if (VD->getName().empty())
+          continue;
+        topLevelDecls[VD->getName()]
+          .push_back({ getKindForTable(D), addDeclRef(D) });
 
-    } else if (auto ED = dyn_cast<ExtensionDecl>(D)) {
-      Type extendedTy = ED->getExtendedType();
-      const NominalTypeDecl *extendedNominal = extendedTy->getAnyNominal();
-      extensionDecls[extendedNominal->getName()]
-        .push_back({ getKindForTable(extendedNominal), addDeclRef(D) });
+      } else if (auto ED = dyn_cast<ExtensionDecl>(D)) {
+        Type extendedTy = ED->getExtendedType();
+        const NominalTypeDecl *extendedNominal = extendedTy->getAnyNominal();
+        extensionDecls[extendedNominal->getName()]
+          .push_back({ getKindForTable(extendedNominal), addDeclRef(D) });
 
-    } else if (auto OD = dyn_cast<OperatorDecl>(D)) {
-      operatorDecls[OD->getName()]
-        .push_back({ getStableFixity(OD->getKind()), addDeclRef(D) });
+      } else if (auto OD = dyn_cast<OperatorDecl>(D)) {
+        operatorDecls[OD->getName()]
+          .push_back({ getStableFixity(OD->getKind()), addDeclRef(D) });
+      }
     }
   }
 
diff --git a/tools/swift/Immediate.cpp b/tools/swift/Immediate.cpp
index fa2598f3525..2edad58a507 100644
--- a/tools/swift/Immediate.cpp
+++ b/tools/swift/Immediate.cpp
@@ -1025,7 +1025,7 @@ public:
   REPLEnvironment(CompilerInstance &CI,
                   bool ShouldRunREPLApplicationMain,
                   const ProcessCmdLine &CmdLine)
-    : CI(CI), REPLInputFile(*CI.getTU()->MainSourceFile),
+    : CI(CI), REPLInputFile(CI.getTU()->getMainSourceFile(SourceFile::REPL)),
       ShouldRunREPLApplicationMain(ShouldRunREPLApplicationMain),
       CmdLine(CmdLine),
       RanGlobalInitializers(false),