diff --git a/lib/Serialization/ModuleFile.cpp b/lib/Serialization/ModuleFile.cpp
index dfe2e5e769c..1f51ae8b928 100644
--- a/lib/Serialization/ModuleFile.cpp
+++ b/lib/Serialization/ModuleFile.cpp
@@ -896,6 +896,71 @@ Decl *ModuleFile::getDecl(DeclID DID, Optional<DeclContext *> ForcedContext) {
     break;
   }
 
+  case decls_block::ONEOF_DECL: {
+    IdentifierID nameID;
+    DeclID contextID;
+    bool isImplicit;
+    ArrayRef<uint64_t> inheritedIDs;
+
+    decls_block::OneOfLayout::readRecord(scratch, nameID, contextID,
+                                         isImplicit, inheritedIDs);
+
+    MutableArrayRef<TypeLoc> inherited;
+    DeclContext *DC;
+    {
+      BCOffsetRAII restoreOffset(DeclTypeCursor);
+      inherited = getTypes(inheritedIDs);
+      DC = getDeclContext(contextID);
+    }
+
+    auto genericParams = maybeReadGenericParams(DC);
+
+    auto oneOf = new (ctx) OneOfDecl(SourceLoc(), getIdentifier(nameID),
+                                     SourceLoc(), inherited,
+                                     genericParams, DC);
+    declOrOffset = oneOf;
+
+    if (isImplicit)
+      oneOf->setImplicit();
+    if (genericParams)
+      for (auto &genericParam : *oneOf->getGenericParams())
+        genericParam.getAsTypeParam()->setDeclContext(oneOf);
+
+    SmallVector<ProtocolConformance *, 16> conformanceBuf;
+    while (ProtocolConformance *conformance = maybeReadConformance())
+      conformanceBuf.push_back(conformance);
+    oneOf->setConformances(ctx.AllocateCopy(conformanceBuf));
+
+    auto members = readMembers();
+    assert(members.hasValue() && "could not read oneof members");
+    oneOf->setMembers(members.getValue(), SourceRange());
+
+    break;
+  }
+
+  case decls_block::ONEOF_ELEMENT_DECL: {
+    IdentifierID nameID;
+    DeclID contextID;
+    TypeID argTypeID, ctorTypeID;
+    bool isImplicit;
+
+    decls_block::OneOfElementLayout::readRecord(scratch, nameID, contextID,
+                                                argTypeID, ctorTypeID,
+                                                isImplicit);
+
+    auto argTy = getType(argTypeID);
+    auto elem = new (ctx) OneOfElementDecl(SourceLoc(), getIdentifier(nameID),
+                                           TypeLoc::withoutLoc(argTy),
+                                           getDeclContext(contextID));
+    declOrOffset = elem;
+
+    elem->setType(getType(ctorTypeID));
+    if (isImplicit)
+      elem->setImplicit();
+
+    break;
+  }
+
   case decls_block::XREF: {
     uint8_t kind;
     TypeID expectedTypeID;
diff --git a/lib/Serialization/ModuleFormat.h b/lib/Serialization/ModuleFormat.h
index 16ca6391144..8aad4536634 100644
--- a/lib/Serialization/ModuleFormat.h
+++ b/lib/Serialization/ModuleFormat.h
@@ -237,6 +237,8 @@ namespace decls_block {
     POSTFIX_OPERATOR_DECL,
     INFIX_OPERATOR_DECL,
     CLASS_DECL,
+    ONEOF_DECL,
+    ONEOF_ELEMENT_DECL,
 
     PAREN_PATTERN = 200,
     TUPLE_PATTERN,
@@ -406,6 +408,7 @@ namespace decls_block {
   using StructLayout = NominalLayout<STRUCT_DECL>;
   using ProtocolLayout = NominalLayout<PROTOCOL_DECL>;
   using ClassLayout = NominalLayout<CLASS_DECL>;
+  using OneOfLayout = NominalLayout<ONEOF_DECL>;
 
   using ConstructorLayout = BCRecordLayout<
     CONSTRUCTOR_DECL,
@@ -466,6 +469,15 @@ namespace decls_block {
     BCFixed<8>   // precedence
   >;
 
+  using OneOfElementLayout = BCRecordLayout<
+    ONEOF_ELEMENT_DECL,
+    IdentifierIDField, // name
+    DeclIDField, // context decl
+    TypeIDField, // argument type
+    TypeIDField, // constructor type
+    BCFixed<1>   // implicit?
+  >;
+
 
   using ParenPatternLayout = BCRecordLayout<
     PAREN_PATTERN
diff --git a/lib/Serialization/Serialization.cpp b/lib/Serialization/Serialization.cpp
index 66f9fe44eba..fe95764a95d 100644
--- a/lib/Serialization/Serialization.cpp
+++ b/lib/Serialization/Serialization.cpp
@@ -424,6 +424,8 @@ void Serializer::writeBlockInfoBlock() {
   RECORD(decls_block, POSTFIX_OPERATOR_DECL);
   RECORD(decls_block, INFIX_OPERATOR_DECL);
   RECORD(decls_block, CLASS_DECL);
+  RECORD(decls_block, ONEOF_DECL);
+  RECORD(decls_block, ONEOF_ELEMENT_DECL);
 
   RECORD(decls_block, PAREN_PATTERN);
   RECORD(decls_block, TUPLE_PATTERN);
@@ -888,8 +890,31 @@ bool Serializer::writeDecl(const Decl *D) {
     return true;
   }
 
-  case DeclKind::OneOf:
-    return false;
+  case DeclKind::OneOf: {
+    auto oneOf = cast<OneOfDecl>(D);
+
+    // FIXME: Handle attributes.
+    if (!oneOf->getAttrs().empty())
+      return false;
+
+    const Decl *DC = getDeclForContext(oneOf->getDeclContext());
+
+    SmallVector<TypeID, 4> inherited;
+    for (auto parent : oneOf->getInherited())
+      inherited.push_back(addTypeRef(parent.getType()));
+
+    unsigned abbrCode = DeclTypeAbbrCodes[OneOfLayout::Code];
+    OneOfLayout::emitRecord(Out, ScratchRecord, abbrCode,
+                            addIdentifierRef(oneOf->getName()),
+                            addDeclRef(DC),
+                            oneOf->isImplicit(),
+                            inherited);
+
+    writeGenericParams(oneOf->getGenericParams());
+    writeConformances(oneOf->getConformances());
+    writeMembers(oneOf);
+    return true;
+  }
 
   case DeclKind::Class: {
     auto theClass = cast<ClassDecl>(D);
@@ -1006,7 +1031,25 @@ bool Serializer::writeDecl(const Decl *D) {
     return true;
   }
 
-  case DeclKind::OneOfElement:
+  case DeclKind::OneOfElement: {
+    auto elem = cast<OneOfElementDecl>(D);
+
+    // FIXME: Handle attributes.
+    if (!elem->getAttrs().empty())
+      return false;
+
+    const Decl *DC = getDeclForContext(elem->getDeclContext());
+
+    unsigned abbrCode = DeclTypeAbbrCodes[OneOfElementLayout::Code];
+    OneOfElementLayout::emitRecord(Out, ScratchRecord, abbrCode,
+                                   addIdentifierRef(elem->getName()),
+                                   addDeclRef(DC),
+                                   addTypeRef(elem->getArgumentType()),
+                                   addTypeRef(elem->getType()),
+                                   elem->isImplicit());
+    return true;
+  }
+
   case DeclKind::Subscript:
     return false;
 
@@ -1370,6 +1413,8 @@ void Serializer::writeAllDeclsAndTypes() {
     registerDeclTypeAbbr<PostfixOperatorLayout>();
     registerDeclTypeAbbr<InfixOperatorLayout>();
     registerDeclTypeAbbr<ClassLayout>();
+    registerDeclTypeAbbr<OneOfLayout>();
+    registerDeclTypeAbbr<OneOfElementLayout>();
 
     registerDeclTypeAbbr<ParenPatternLayout>();
     registerDeclTypeAbbr<TuplePatternLayout>();
diff --git a/test/Serialization/Inputs/def_oneof.swift b/test/Serialization/Inputs/def_oneof.swift
new file mode 100644
index 00000000000..d3afb843137
--- /dev/null
+++ b/test/Serialization/Inputs/def_oneof.swift
@@ -0,0 +1,30 @@
+oneof Basic {
+  Untyped,
+  HasType : Int
+  
+  constructor() {
+    this = .Untyped
+  }
+  func doSomething() {}
+}
+
+oneof Generic<A> {
+  Left : A,
+  Right : A
+}
+
+
+protocol Computable {
+  func compute()
+}
+
+oneof Lazy<T> : Computable {
+  Thunk : () -> T,
+  Value : T
+  
+  func compute() {
+//    if (this ~= .Thunk(var fn)) {
+//      this = .Value(fn())
+//    }
+  }
+}
diff --git a/test/Serialization/oneof.swift b/test/Serialization/oneof.swift
new file mode 100644
index 00000000000..2cff408e368
--- /dev/null
+++ b/test/Serialization/oneof.swift
@@ -0,0 +1,23 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: %swift -emit-module -o %t/def_oneof.swiftmodule %S/Inputs/def_oneof.swift
+// RUN: llvm-bcanalyzer %t/def_oneof.swiftmodule | FileCheck %s
+// RUN: %swift -parse -I=%t %s -o /dev/null
+
+// CHECK-NOT: FALL_BACK_TO_TRANSLATION_UNIT
+
+import def_oneof
+
+var a : Basic
+a = .Untyped
+a.doSomething()
+a = .HasType(4)
+a.doSomething()
+
+var g = Generic.Left(false)
+g = .Right(true)
+
+var lazy = Lazy.Thunk({ 42 })
+var comp : Computable = lazy
+comp.compute()
+lazy.compute()