diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 50cc781efc..130188ef98 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -8735,6 +8735,25 @@ nsContentUtils::PushEnabled(JSContext* aCx, JSObject* aObj)
   return workerPrivate->PushEnabled();
 }
 
+// static
+bool
+nsContentUtils::StreamsEnabled(JSContext* aCx, JSObject* aObj)
+{
+  if (NS_IsMainThread()) {
+    return Preferences::GetBool("dom.streams.enabled", false);
+  }
+
+  using namespace workers;
+
+  // Otherwise, check the pref via the WorkerPrivate
+  WorkerPrivate* workerPrivate = GetWorkerPrivateFromContext(aCx);
+  if (!workerPrivate) {
+    return false;
+  }
+
+  return workerPrivate->StreamsEnabled();
+}
+
 // static
 bool
 nsContentUtils::IsNonSubresourceRequest(nsIChannel* aChannel)
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index c0e44b3be4..c1f8885f73 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -2679,6 +2679,8 @@ public:
   static mozilla::net::ReferrerPolicy GetReferrerPolicyFromHeader(const nsAString& aHeader);
 
   static bool PushEnabled(JSContext* aCx, JSObject* aObj);
+  
+  static bool StreamsEnabled(JSContext* aCx, JSObject* aObj);
 
   static bool IsNonSubresourceRequest(nsIChannel* aChannel);
 
diff --git a/dom/bindings/Bindings.conf b/dom/bindings/Bindings.conf
index e1560bfa3f..39fa0d7dc9 100644
--- a/dom/bindings/Bindings.conf
+++ b/dom/bindings/Bindings.conf
@@ -722,6 +722,7 @@ DOMInterfaces = {
         'headers': 'headers_',
         'referrerPolicy': 'referrerPolicy_'
     },
+    'implicitJSContext': [ 'arrayBuffer', 'blob', 'formData', 'json', 'text' ],
 },
 
 'ResizeObservation': {
@@ -746,6 +747,7 @@ DOMInterfaces = {
 
 'Response': {
     'binaryNames': { 'headers': 'headers_' },
+    'implicitJSContext': [ 'arrayBuffer', 'blob', 'formData', 'json', 'text' ],
 },
 
 'RGBColor': {
diff --git a/dom/fetch/Fetch.cpp b/dom/fetch/Fetch.cpp
index 42c9fc74f9..63e2c83eaa 100644
--- a/dom/fetch/Fetch.cpp
+++ b/dom/fetch/Fetch.cpp
@@ -5,6 +5,7 @@
 
 #include "Fetch.h"
 #include "FetchConsumer.h"
+#include "FetchStream.h"
 
 #include "nsIDocument.h"
 #include "nsIGlobalObject.h"
@@ -22,6 +23,7 @@
 
 #include "mozilla/ErrorResult.h"
 #include "mozilla/dom/BodyUtil.h"
+#include "mozilla/dom/DOMError.h"
 #include "mozilla/dom/EncodingUtils.h"
 #include "mozilla/dom/Exceptions.h"
 #include "mozilla/dom/FetchDriver.h"
@@ -35,6 +37,7 @@
 #include "mozilla/dom/Response.h"
 #include "mozilla/dom/ScriptSettings.h"
 #include "mozilla/dom/URLSearchParams.h"
+#include "mozilla/dom/WorkerPrivate.h"
 #include "mozilla/dom/workers/ServiceWorkerManager.h"
 
 #include "FetchObserver.h"
@@ -918,6 +921,7 @@ ExtractByteStreamFromBody(const ArrayBufferOrArrayBufferViewOrBlobOrFormDataOrUS
 template <class Derived>
 FetchBody<Derived>::FetchBody()
   : mWorkerPrivate(nullptr)
+  , mReadableStreamBody(nullptr)
   , mBodyUsed(false)
 {
   if (!NS_IsMainThread()) {
@@ -937,9 +941,45 @@ FetchBody<Derived>::~FetchBody()
 {
 }
 
+template <class Derived>
+bool
+FetchBody<Derived>::BodyUsed() const
+{
+  if (mBodyUsed) {
+    return true;
+  }
+
+  // If this object is disturbed or locked, return false.
+  if (mReadableStreamBody) {
+    AutoJSAPI jsapi;
+    // If we start tracking ownership, we need something like this.
+    //if (!jsapi.Init(mOwner)) {
+    //  return true;
+    //}
+
+    JSContext* cx = jsapi.cx();
+
+    JS::Rooted<JSObject*> body(cx, mReadableStreamBody);
+    if (JS::ReadableStreamIsDisturbed(body) ||
+        JS::ReadableStreamIsLocked(body)) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+template
+bool
+FetchBody<Request>::BodyUsed() const;
+
+template
+bool
+FetchBody<Response>::BodyUsed() const;
+
 template <class Derived>
 already_AddRefed<Promise>
-FetchBody<Derived>::ConsumeBody(FetchConsumeType aType, ErrorResult& aRv)
+FetchBody<Derived>::ConsumeBody(JSContext* aCx, FetchConsumeType aType, ErrorResult& aRv)
 {
   RefPtr<AbortSignal> signal = DerivedClass()->GetSignal();
   if (signal && signal->Aborted()) {
@@ -954,9 +994,16 @@ FetchBody<Derived>::ConsumeBody(FetchConsumeType aType, ErrorResult& aRv)
 
   SetBodyUsed();
 
+  nsCOMPtr<nsIGlobalObject> global = DerivedClass()->GetParentObject();
+
+  // If we already created a ReadableStreamBody we have to close it now.
+  if (mReadableStreamBody) {
+    JS::Rooted<JSObject*> body(aCx, mReadableStreamBody);
+    JS::ReadableStreamClose(aCx, body);
+  }
+
   RefPtr<Promise> promise =
-    FetchBodyConsumer<Derived>::Create(DerivedClass()->GetParentObject(),
-                                       this, signal, aType, aRv);
+    FetchBodyConsumer<Derived>::Create(global, this, signal, aType, aRv);
   if (NS_WARN_IF(aRv.Failed())) {
     return nullptr;
   }
@@ -966,11 +1013,11 @@ FetchBody<Derived>::ConsumeBody(FetchConsumeType aType, ErrorResult& aRv)
 
 template
 already_AddRefed<Promise>
-FetchBody<Request>::ConsumeBody(FetchConsumeType aType, ErrorResult& aRv);
+FetchBody<Request>::ConsumeBody(JSContext* aCx, FetchConsumeType aType, ErrorResult& aRv);
 
 template
 already_AddRefed<Promise>
-FetchBody<Response>::ConsumeBody(FetchConsumeType aType, ErrorResult& aRv);
+FetchBody<Response>::ConsumeBody(JSContext* aCx, FetchConsumeType aType, ErrorResult& aRv);
 
 template <class Derived>
 void
@@ -1003,20 +1050,52 @@ FetchBody<Response>::SetMimeType();
 template <class Derived>
 void
 FetchBody<Derived>::GetBody(JSContext* aCx,
-                            JS::MutableHandle<JSObject*> aMessage)
+                            JS::MutableHandle<JSObject*> aBodyOut,
+                            ErrorResult& aRv)
 {
-  // TODO
+  nsCOMPtr<nsIInputStream> inputStream;
+  DerivedClass()->GetBody(getter_AddRefs(inputStream));
+
+  if (!inputStream) {
+    aBodyOut.set(nullptr);
+    return;
+  }
+
+  if (!mReadableStreamBody) {
+    JS::Rooted<JSObject*> body(aCx,
+                               FetchStream::Create(aCx,
+                                                   DerivedClass()->GetParentObject(),
+                                                   inputStream,
+                                                   aRv));
+    if (NS_WARN_IF(aRv.Failed())) {
+      return;
+    }
+
+    MOZ_ASSERT(body);
+
+    // If the body has been already consumed, we close the stream.
+    if (BodyUsed() && !JS::ReadableStreamClose(aCx, body)) {
+      aRv.StealExceptionFromJSContext(aCx);
+      return;
+    }
+
+    mReadableStreamBody = body;
+  }
+
+  aBodyOut.set(mReadableStreamBody);
 }
 
 template
 void
 FetchBody<Request>::GetBody(JSContext* aCx,
-                            JS::MutableHandle<JSObject*> aMessage);
+                            JS::MutableHandle<JSObject*> aMessage,
+                            ErrorResult& aRv);
 
 template
 void
 FetchBody<Response>::GetBody(JSContext* aCx,
-                             JS::MutableHandle<JSObject*> aMessage);
+                             JS::MutableHandle<JSObject*> aMessage,
+                             ErrorResult& aRv);
 
 
 } // namespace dom
diff --git a/dom/fetch/Fetch.h b/dom/fetch/Fetch.h
index bc2df62689..821bc3643f 100644
--- a/dom/fetch/Fetch.h
+++ b/dom/fetch/Fetch.h
@@ -115,41 +115,42 @@ public:
   NS_IMETHOD_(MozExternalRefCountType) Release(void) = 0;
 
   bool
-  BodyUsed() const { return mBodyUsed; }
+  BodyUsed() const;
 
   already_AddRefed<Promise>
-  ArrayBuffer(ErrorResult& aRv)
+  ArrayBuffer(JSContext* aCx, ErrorResult& aRv)
   {
-    return ConsumeBody(CONSUME_ARRAYBUFFER, aRv);
+    return ConsumeBody(aCx, CONSUME_ARRAYBUFFER, aRv);
   }
 
   already_AddRefed<Promise>
-  Blob(ErrorResult& aRv)
+  Blob(JSContext* aCx, ErrorResult& aRv)
   {
-    return ConsumeBody(CONSUME_BLOB, aRv);
+    return ConsumeBody(aCx, CONSUME_BLOB, aRv);
   }
 
   already_AddRefed<Promise>
-  FormData(ErrorResult& aRv)
+  FormData(JSContext* aCx, ErrorResult& aRv)
   {
-    return ConsumeBody(CONSUME_FORMDATA, aRv);
+    return ConsumeBody(aCx, CONSUME_FORMDATA, aRv);
   }
 
   already_AddRefed<Promise>
-  Json(ErrorResult& aRv)
+  Json(JSContext* aCx, ErrorResult& aRv)
   {
-    return ConsumeBody(CONSUME_JSON, aRv);
+    return ConsumeBody(aCx, CONSUME_JSON, aRv);
   }
 
   already_AddRefed<Promise>
-  Text(ErrorResult& aRv)
+  Text(JSContext* aCx, ErrorResult& aRv)
   {
-    return ConsumeBody(CONSUME_TEXT, aRv);
+    return ConsumeBody(aCx, CONSUME_TEXT, aRv);
   }
 
   void
   GetBody(JSContext* aCx,
-          JS::MutableHandle<JSObject*> aMessage);
+          JS::MutableHandle<JSObject*> aBodyOut,
+          ErrorResult& aRv);
 
   // Utility public methods accessed by various runnables.
 
@@ -174,6 +175,9 @@ protected:
   // Always set whenever the FetchBody is created on the worker thread.
   workers::WorkerPrivate* mWorkerPrivate;
 
+  // This is the ReadableStream exposed to content. Its underlying source is a FetchStream object.
+  JS::Heap<JSObject*> mReadableStreamBody;
+
   virtual ~FetchBody();
 
   void
@@ -186,7 +190,7 @@ private:
   }
 
   already_AddRefed<Promise>
-  ConsumeBody(FetchConsumeType aType, ErrorResult& aRv);
+  ConsumeBody(JSContext* aCx, FetchConsumeType aType, ErrorResult& aRv);
 
   bool
   IsOnTargetThread()
diff --git a/dom/fetch/FetchStream.cpp b/dom/fetch/FetchStream.cpp
new file mode 100644
index 0000000000..7b165b2a9d
--- /dev/null
+++ b/dom/fetch/FetchStream.cpp
@@ -0,0 +1,336 @@
+/* -*- Mode: C; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "FetchStream.h"
+#include "nsNetCID.h"
+#include "nsITransport.h"
+#include "nsIStreamTransportService.h"
+#include "nsProxyRelease.h"
+
+#include "mozilla/dom/DOMError.h"
+
+#define FETCH_STREAM_FLAG 0
+
+static NS_DEFINE_CID(kStreamTransportServiceCID,
+                     NS_STREAMTRANSPORTSERVICE_CID);
+
+namespace mozilla {
+namespace dom {
+
+NS_IMPL_ISUPPORTS(FetchStream, nsIInputStreamCallback)
+
+/* static */ JSObject*
+FetchStream::Create(JSContext* aCx, nsIGlobalObject* aGlobal,
+                    nsIInputStream* aInputStream, ErrorResult& aRv)
+{
+  MOZ_DIAGNOSTIC_ASSERT(aCx);
+  MOZ_DIAGNOSTIC_ASSERT(aInputStream);
+
+  RefPtr<FetchStream> stream = new FetchStream(aGlobal, aInputStream);
+
+  if (!JS::HasReadableStreamCallbacks(aCx)) {
+    JS::SetReadableStreamCallbacks(aCx,
+                                   &FetchStream::RequestDataCallback,
+                                   &FetchStream::WriteIntoReadRequestCallback,
+                                   &FetchStream::CancelCallback,
+                                   &FetchStream::ClosedCallback,
+                                   &FetchStream::ErroredCallback,
+                                   &FetchStream::FinalizeCallback);
+  }
+
+  JS::Rooted<JSObject*> body(aCx,
+    JS::NewReadableExternalSourceStreamObject(aCx, stream, FETCH_STREAM_FLAG));
+  if (!body) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return nullptr;
+  }
+
+  stream->mReadableStream = body;
+
+  // JS engine will call the finalize callback.
+  NS_ADDREF(stream.get());
+  return body;
+}
+
+/* static */ void
+FetchStream::RequestDataCallback(JSContext* aCx,
+                                 JS::HandleObject aStream,
+                                 void* aUnderlyingSource,
+                                 uint8_t aFlags,
+                                 size_t aDesiredSize)
+{
+  MOZ_DIAGNOSTIC_ASSERT(aUnderlyingSource);
+  MOZ_DIAGNOSTIC_ASSERT(aFlags == FETCH_STREAM_FLAG);
+  MOZ_DIAGNOSTIC_ASSERT(JS::ReadableStreamIsDisturbed(aStream));
+
+  RefPtr<FetchStream> stream = static_cast<FetchStream*>(aUnderlyingSource);
+
+  MOZ_DIAGNOSTIC_ASSERT(stream->mState == eWaiting ||
+                        stream->mState == eChecking);
+
+  if (stream->mState == eChecking) {
+    // If we are looking for more data, there is nothing else we should do:
+    // let's move this checking operation in a reading.
+    MOZ_ASSERT(stream->mInputStream);
+    stream->mState = eReading;
+    return;
+  }
+
+  stream->mState = eReading;
+
+  if (!stream->mInputStream) {
+    // This is the first use of the stream. Let's convert the
+    // mOriginalInputStream into an nsIAsyncInputStream.
+    MOZ_ASSERT(stream->mOriginalInputStream);
+
+    bool nonBlocking = false;
+    nsresult rv = stream->mOriginalInputStream->IsNonBlocking(&nonBlocking);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      stream->ErrorPropagation(aCx, aStream, rv);
+      return;
+    }
+
+    nsCOMPtr<nsIAsyncInputStream> asyncStream =
+      do_QueryInterface(stream->mOriginalInputStream);
+    if (!nonBlocking || !asyncStream) {
+      nsCOMPtr<nsIStreamTransportService> sts =
+        do_GetService(kStreamTransportServiceCID, &rv);
+      if (NS_WARN_IF(NS_FAILED(rv))) {
+        stream->ErrorPropagation(aCx, aStream, rv);
+        return;
+      }
+
+      nsCOMPtr<nsITransport> transport;
+      rv = sts->CreateInputTransport(stream->mOriginalInputStream,
+                                     /* aStartOffset */ 0,
+                                     /* aReadLimit */ -1,
+                                     /* aCloseWhenDone */ true,
+                                     getter_AddRefs(transport));
+      if (NS_WARN_IF(NS_FAILED(rv))) {
+        stream->ErrorPropagation(aCx, aStream, rv);
+        return;
+      }
+
+      nsCOMPtr<nsIInputStream> wrapper;
+      rv = transport->OpenInputStream(/* aFlags */ 0,
+                                       /* aSegmentSize */ 0,
+                                       /* aSegmentCount */ 0,
+                                       getter_AddRefs(wrapper));
+      if (NS_WARN_IF(NS_FAILED(rv))) {
+        stream->ErrorPropagation(aCx, aStream, rv);
+        return;
+      }
+
+      asyncStream = do_QueryInterface(wrapper);
+    }
+
+    stream->mInputStream = asyncStream;
+    stream->mOriginalInputStream = nullptr;
+  }
+
+  MOZ_DIAGNOSTIC_ASSERT(stream->mInputStream);
+  MOZ_DIAGNOSTIC_ASSERT(!stream->mOriginalInputStream);
+
+  nsresult rv =
+    stream->mInputStream->AsyncWait(stream, 0, 0, nullptr);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    stream->ErrorPropagation(aCx, aStream, rv);
+    return;
+  }
+
+  // All good.
+}
+
+/* static */ void
+FetchStream::WriteIntoReadRequestCallback(JSContext* aCx,
+                                          JS::HandleObject aStream,
+                                          void* aUnderlyingSource,
+                                          uint8_t aFlags, void* aBuffer,
+                                          size_t aLength, size_t* aByteWritten)
+{
+  MOZ_DIAGNOSTIC_ASSERT(aUnderlyingSource);
+  MOZ_DIAGNOSTIC_ASSERT(aFlags == FETCH_STREAM_FLAG);
+  MOZ_DIAGNOSTIC_ASSERT(aBuffer);
+  MOZ_DIAGNOSTIC_ASSERT(aByteWritten);
+
+  RefPtr<FetchStream> stream = static_cast<FetchStream*>(aUnderlyingSource);
+
+  MOZ_DIAGNOSTIC_ASSERT(stream->mInputStream);
+  MOZ_DIAGNOSTIC_ASSERT(stream->mState == eWriting);
+  stream->mState = eChecking;
+
+  uint32_t written;
+  nsresult rv =
+    stream->mInputStream->Read(static_cast<char*>(aBuffer), aLength, &written);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    stream->ErrorPropagation(aCx, aStream, rv);
+    return;
+  }
+
+  *aByteWritten = written;
+
+  if (written == 0) {
+    stream->mState = eClosed;
+    JS::ReadableStreamClose(aCx, aStream);
+    return;
+  }
+
+  rv = stream->mInputStream->AsyncWait(stream, 0, 0, nullptr);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    stream->ErrorPropagation(aCx, aStream, rv);
+    return;
+  }
+
+  // All good.
+}
+
+/* static */ JS::Value
+FetchStream::CancelCallback(JSContext* aCx, JS::HandleObject aStream,
+                            void* aUnderlyingSource, uint8_t aFlags,
+                            JS::HandleValue aReason)
+{
+  MOZ_DIAGNOSTIC_ASSERT(aUnderlyingSource);
+  MOZ_DIAGNOSTIC_ASSERT(aFlags == FETCH_STREAM_FLAG);
+
+  RefPtr<FetchStream> stream = static_cast<FetchStream*>(aUnderlyingSource);
+
+  if (stream->mInputStream) {
+    stream->mInputStream->CloseWithStatus(NS_BASE_STREAM_CLOSED);
+  }
+
+  stream->mState = eClosed;
+
+  return JS::UndefinedValue();
+}
+
+/* static */ void
+FetchStream::ClosedCallback(JSContext* aCx, JS::HandleObject aStream,
+                            void* aUnderlyingSource, uint8_t aFlags)
+{
+  MOZ_DIAGNOSTIC_ASSERT(aUnderlyingSource);
+  MOZ_DIAGNOSTIC_ASSERT(aFlags == FETCH_STREAM_FLAG);
+}
+
+/* static */ void
+FetchStream::ErroredCallback(JSContext* aCx, JS::HandleObject aStream,
+                             void* aUnderlyingSource, uint8_t aFlags,
+                             JS::HandleValue aReason)
+{
+  MOZ_DIAGNOSTIC_ASSERT(aUnderlyingSource);
+  MOZ_DIAGNOSTIC_ASSERT(aFlags == FETCH_STREAM_FLAG);
+}
+
+void
+FetchStream::FinalizeCallback(void* aUnderlyingSource, uint8_t aFlags)
+{
+  MOZ_DIAGNOSTIC_ASSERT(aUnderlyingSource);
+  MOZ_DIAGNOSTIC_ASSERT(aFlags == FETCH_STREAM_FLAG);
+
+  RefPtr<FetchStream> stream =
+    dont_AddRef(static_cast<FetchStream*>(aUnderlyingSource));
+
+  stream->mState = eClosed;
+  stream->mReadableStream = nullptr;
+}
+
+FetchStream::FetchStream(nsIGlobalObject* aGlobal,
+                         nsIInputStream* aInputStream)
+  : mState(eWaiting)
+  , mGlobal(aGlobal)
+  , mOriginalInputStream(aInputStream)
+  , mOwningEventTarget(nullptr)
+  , mReadableStream(nullptr)
+{
+  MOZ_DIAGNOSTIC_ASSERT(aInputStream);
+}
+
+FetchStream::~FetchStream()
+{
+  NS_ProxyRelease(mOwningEventTarget, mGlobal.forget());
+}
+
+void
+FetchStream::ErrorPropagation(JSContext* aCx, JS::HandleObject aStream,
+                              nsresult aError)
+{
+  // Nothing to do.
+  if (mState == eClosed) {
+    return;
+  }
+
+  // We cannot continue with any other operation.
+  mState = eClosed;
+
+  // Let's close the stream.
+  if (aError == NS_BASE_STREAM_CLOSED) {
+    JS::ReadableStreamClose(aCx, aStream);
+    return;
+  }
+
+  nsCOMPtr<nsPIDOMWindowInner> window = do_QueryInterface(mGlobal);
+
+  // Let's use a generic error.
+  RefPtr<DOMError> error = new DOMError(window, NS_ERROR_DOM_TYPE_ERR);
+
+  JS::Rooted<JS::Value> errorValue(aCx);
+  if (ToJSValue(aCx, error, &errorValue)) {
+    JS::ReadableStreamError(aCx, aStream, errorValue);
+  }
+}
+
+NS_IMETHODIMP
+FetchStream::OnInputStreamReady(nsIAsyncInputStream* aStream)
+{
+  MOZ_DIAGNOSTIC_ASSERT(aStream);
+
+  // Already closed. We have nothing else to do here.
+  if (mState == eClosed) {
+    return NS_OK;
+  }
+
+  MOZ_DIAGNOSTIC_ASSERT(mInputStream);
+  MOZ_DIAGNOSTIC_ASSERT(mState == eReading || mState == eChecking);
+
+  AutoJSAPI jsapi;
+  if (NS_WARN_IF(!jsapi.Init(mGlobal))) {
+    // Without JSContext we are not able to close the stream or to propagate the
+    // error.
+    return NS_ERROR_FAILURE;
+  }
+
+  JSContext* cx = jsapi.cx();
+  JS::Rooted<JSObject*> stream(cx, mReadableStream);
+
+  uint64_t size = 0;
+  nsresult rv = mInputStream->Available(&size);
+  if (NS_SUCCEEDED(rv) && size == 0) {
+    // In theory this should not happen. If size is 0, the stream should be
+    // considered closed.
+    rv = NS_BASE_STREAM_CLOSED;
+  }
+
+  // No warning for stream closed.
+  if (rv == NS_BASE_STREAM_CLOSED || NS_WARN_IF(NS_FAILED(rv))) {
+    ErrorPropagation(cx, stream, rv);
+    return NS_OK;
+  }
+
+  // This extra checking is completed. Let's wait for the next read request.
+  if (mState == eChecking) {
+    mState = eWaiting;
+    return NS_OK;
+  }
+
+  mState = eWriting;
+  JS::ReadableStreamUpdateDataAvailableFromSource(cx, stream, size);
+
+  // The WriteInto callback changes mState to eChecking.
+  MOZ_DIAGNOSTIC_ASSERT(mState == eChecking);
+
+  return NS_OK;
+}
+
+} // dom namespace
+} // mozilla namespace
diff --git a/dom/fetch/FetchStream.h b/dom/fetch/FetchStream.h
new file mode 100644
index 0000000000..ebe94e205b
--- /dev/null
+++ b/dom/fetch/FetchStream.h
@@ -0,0 +1,107 @@
+/* -*- Mode: C; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_FetchStream_h
+#define mozilla_dom_FetchStream_h
+
+#include "Fetch.h"
+#include "jsapi.h"
+#include "nsIAsyncInputStream.h"
+#include "nsISupportsImpl.h"
+
+class nsIGlobalObject;
+
+class nsIInputStream;
+
+namespace mozilla {
+namespace dom {
+
+class FetchStream final : public nsIInputStreamCallback
+{
+public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIINPUTSTREAMCALLBACK
+
+  static JSObject*
+  Create(JSContext* aCx, nsIGlobalObject* aGlobal,
+         nsIInputStream* aInputStream, ErrorResult& aRv);
+
+private:
+  FetchStream(nsIGlobalObject* aGlobal, nsIInputStream* aInputStream);
+  ~FetchStream();
+
+  static void
+  RequestDataCallback(JSContext* aCx, JS::HandleObject aStream,
+                      void* aUnderlyingSource, uint8_t aFlags,
+                      size_t aDesiredSize);
+
+  static void
+  WriteIntoReadRequestCallback(JSContext* aCx, JS::HandleObject aStream,
+                               void* aUnderlyingSource, uint8_t aFlags,
+                               void* aBuffer, size_t aLength,
+                               size_t* aByteWritten);
+
+  static JS::Value
+  CancelCallback(JSContext* aCx, JS::HandleObject aStream,
+                 void* aUnderlyingSource, uint8_t aFlags,
+                 JS::HandleValue aReason);
+
+  static void
+  ClosedCallback(JSContext* aCx, JS::HandleObject aStream,
+                 void* aUnderlyingSource, uint8_t aFlags);
+
+  static void
+  ErroredCallback(JSContext* aCx, JS::HandleObject aStream,
+                  void* aUnderlyingSource, uint8_t aFlags,
+                  JS::HandleValue reason);
+
+  static void
+  FinalizeCallback(void* aUnderlyingSource, uint8_t aFlags);
+
+  void
+  ErrorPropagation(JSContext* aCx, JS::HandleObject aStream, nsresult aRv);
+
+  // Common methods
+
+  enum State {
+    // RequestDataCallback has not been called yet. We haven't started to read
+    // data from the stream yet.
+    eWaiting,
+
+    // We are reading data in a separate I/O thread.
+    eReading,
+
+    // We are ready to write something in the JS Buffer.
+    eWriting,
+
+    // After a writing, we want to check if the stream is closed. After the
+    // check, we go back to eWaiting. If a reading request happens in the
+    // meantime, we move to eReading state.
+    eChecking,
+
+    // Operation completed.
+    eClosed,
+  };
+
+  // Touched only on the target thread.
+  State mState;
+
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+
+  // This is the original inputStream received during the CTOR. It will be
+  // converted into an nsIAsyncInputStream and stored into mInputStream at the
+  // first use.
+  nsCOMPtr<nsIInputStream> mOriginalInputStream;
+  nsCOMPtr<nsIAsyncInputStream> mInputStream;
+
+  nsCOMPtr<nsIEventTarget> mOwningEventTarget;
+
+  JS::Heap<JSObject*> mReadableStream;
+};
+
+} // dom namespace
+} // mozilla namespace
+
+#endif // mozilla_dom_FetchStream_h
diff --git a/dom/fetch/Request.cpp b/dom/fetch/Request.cpp
index ab87a3215a..47fb62e885 100644
--- a/dom/fetch/Request.cpp
+++ b/dom/fetch/Request.cpp
@@ -29,7 +29,24 @@ using namespace workers;
 
 NS_IMPL_CYCLE_COLLECTING_ADDREF(Request)
 NS_IMPL_CYCLE_COLLECTING_RELEASE(Request)
-NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(Request, mOwner, mHeaders)
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(Request)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(Request)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mOwner)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mHeaders)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(Request)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mOwner)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mHeaders)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(Request)
+  NS_IMPL_CYCLE_COLLECTION_TRACE_JS_MEMBER_CALLBACK(mReadableStreamBody)
+  NS_IMPL_CYCLE_COLLECTION_TRACE_PRESERVED_WRAPPER
+NS_IMPL_CYCLE_COLLECTION_TRACE_END
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(Request)
   NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
diff --git a/dom/fetch/Response.cpp b/dom/fetch/Response.cpp
index 42b25ae1d9..20487906ba 100644
--- a/dom/fetch/Response.cpp
+++ b/dom/fetch/Response.cpp
@@ -26,7 +26,27 @@ namespace dom {
 
 NS_IMPL_CYCLE_COLLECTING_ADDREF(Response)
 NS_IMPL_CYCLE_COLLECTING_RELEASE(Response)
-NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(Response, mOwner, mHeaders)
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(Response)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(Response)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mOwner)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mHeaders)
+
+  tmp->mReadableStreamBody = nullptr;
+
+  NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(Response)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mOwner)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mHeaders)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(Response)
+  NS_IMPL_CYCLE_COLLECTION_TRACE_JS_MEMBER_CALLBACK(mReadableStreamBody)
+  NS_IMPL_CYCLE_COLLECTION_TRACE_PRESERVED_WRAPPER
+NS_IMPL_CYCLE_COLLECTION_TRACE_END
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(Response)
   NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
@@ -42,10 +62,14 @@ Response::Response(nsIGlobalObject* aGlobal, InternalResponse* aInternalResponse
   MOZ_ASSERT(aInternalResponse->Headers()->Guard() == HeadersGuardEnum::Immutable ||
              aInternalResponse->Headers()->Guard() == HeadersGuardEnum::Response);
   SetMimeType();
+  
+  // Keep a firm grip!
+  mozilla::HoldJSObjects(this);
 }
 
 Response::~Response()
 {
+  mozilla::DropJSObjects(this);
 }
 
 /* static */ already_AddRefed<Response>
diff --git a/dom/fetch/moz.build b/dom/fetch/moz.build
index d1a8a49323..cd41620f93 100644
--- a/dom/fetch/moz.build
+++ b/dom/fetch/moz.build
@@ -24,6 +24,7 @@ UNIFIED_SOURCES += [
     'FetchConsumer.cpp',
     'FetchDriver.cpp',
     'FetchObserver.cpp',
+    'FetchStream.cpp',
     'FetchUtil.cpp',
     'Headers.cpp',
     'InternalHeaders.cpp',
diff --git a/dom/webidl/Response.webidl b/dom/webidl/Response.webidl
index adaa70d66d..d02b714f7d 100644
--- a/dom/webidl/Response.webidl
+++ b/dom/webidl/Response.webidl
@@ -32,6 +32,7 @@ Response implements Body;
 
 // This should be part of Body but we don't want to expose body to request yet.
 partial interface Response {
+  [GetterThrows, Func="nsContentUtils::StreamsEnabled"]
   readonly attribute ReadableStream? body;
 };
 
diff --git a/dom/workers/WorkerPrefs.h b/dom/workers/WorkerPrefs.h
index 215b375ddb..415435cf06 100644
--- a/dom/workers/WorkerPrefs.h
+++ b/dom/workers/WorkerPrefs.h
@@ -35,6 +35,7 @@ WORKER_SIMPLE_PREF("dom.serviceWorkers.testing.enabled", ServiceWorkersTestingEn
 WORKER_SIMPLE_PREF("dom.serviceWorkers.openWindow.enabled", OpenWindowEnabled, OPEN_WINDOW_ENABLED)
 WORKER_SIMPLE_PREF("dom.storageManager.enabled", StorageManagerEnabled, STORAGEMANAGER_ENABLED)
 WORKER_SIMPLE_PREF("dom.push.enabled", PushEnabled, PUSH_ENABLED)
+WORKER_SIMPLE_PREF("dom.streams.enabled", StreamsEnabled, STREAMS_ENABLED)
 WORKER_SIMPLE_PREF("dom.requestcontext.enabled", RequestContextEnabled, REQUESTCONTEXT_ENABLED)
 WORKER_SIMPLE_PREF("gfx.offscreencanvas.enabled", OffscreenCanvasEnabled, OFFSCREENCANVAS_ENABLED)
 WORKER_SIMPLE_PREF("dom.webkitBlink.dirPicker.enabled", WebkitBlinkDirectoryPickerEnabled, DOM_WEBKITBLINK_DIRPICKER_WEBKITBLINK)
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index d1e2ddf1e2..3d6325655e 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -4585,6 +4585,9 @@ pref("layout.animated-image-layers.enabled", false);
 // Abort API
 pref("dom.abortController.enabled", true);
 
+// Streams API
+pref("dom.streams.enabled", false);
+
 // Push
 pref("dom.push.enabled", false);
 pref("dom.push.loglevel", "error");