diff --git a/dom/media/MediaDecoder.cpp b/dom/media/MediaDecoder.cpp
index cc700f377904..342f21b4ecd5 100644
--- a/dom/media/MediaDecoder.cpp
+++ b/dom/media/MediaDecoder.cpp
@@ -461,7 +461,6 @@ MediaDecoder::MediaDecoder() :
     gMediaDecoderLog = PR_NewLogModule("MediaDecoder");
   }
 #endif
-
   mAudioChannel = AudioChannelService::GetDefaultAudioChannel();
 }
 
diff --git a/dom/media/MediaDecoderReader.cpp b/dom/media/MediaDecoderReader.cpp
index 1de9b6f12580..44ca2d92278d 100644
--- a/dom/media/MediaDecoderReader.cpp
+++ b/dom/media/MediaDecoderReader.cpp
@@ -27,6 +27,16 @@ extern PRLogModuleInfo* gMediaDecoderLog;
 #define DECODER_LOG(x, ...)
 #endif
 
+PRLogModuleInfo* gMediaPromiseLog;
+
+void
+EnsureMediaPromiseLog()
+{
+  if (!gMediaPromiseLog) {
+    gMediaPromiseLog = PR_NewLogModule("MediaPromise");
+  }
+}
+
 class VideoQueueMemoryFunctor : public nsDequeFunctor {
 public:
   VideoQueueMemoryFunctor() : mSize(0) {}
@@ -69,6 +79,7 @@ MediaDecoderReader::MediaDecoderReader(AbstractMediaDecoder* aDecoder)
   , mShutdown(false)
 {
   MOZ_COUNT_CTOR(MediaDecoderReader);
+  EnsureMediaPromiseLog();
 }
 
 MediaDecoderReader::~MediaDecoderReader()
diff --git a/dom/media/MediaPromise.h b/dom/media/MediaPromise.h
new file mode 100644
index 000000000000..f455d5c48b90
--- /dev/null
+++ b/dom/media/MediaPromise.h
@@ -0,0 +1,322 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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/. */
+
+#if !defined(MediaPromise_h_)
+#define MediaPromise_h_
+
+#include "prlog.h"
+
+#include "mozilla/DebugOnly.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/Monitor.h"
+
+#include "MediaTaskQueue.h"
+#include "nsIEventTarget.h"
+
+/* Polyfill __func__ on MSVC for consumers to pass to the MediaPromise API. */
+#ifdef _MSC_VER
+#define __func__ __FUNCTION__
+#endif
+
+namespace mozilla {
+
+extern PRLogModuleInfo* gMediaPromiseLog;
+void EnsureMediaPromiseLog();
+
+#define PROMISE_LOG(x, ...) \
+  MOZ_ASSERT(gMediaPromiseLog); \
+  PR_LOG(gMediaPromiseLog, PR_LOG_DEBUG, (x, ##__VA_ARGS__))
+
+/*
+ * A promise manages an asynchronous request that may or may not be able to be
+ * fulfilled immediately. When an API returns a promise, the consumer may attach
+ * callbacks to be invoked (asynchronously, on a specified thread) when the
+ * request is either completed (resolved) or cannot be completed (rejected).
+ *
+ * By default, resolve and reject callbacks are always invoked on the same thread
+ * where Then() was invoked.
+ */
+template<typename T> class MediaPromiseHolder;
+template<typename ResolveValueT, typename RejectValueT>
+class MediaPromise
+{
+public:
+  typedef ResolveValueT ResolveValueType;
+  typedef RejectValueT RejectValueType;
+
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(MediaPromise)
+  MediaPromise(const char* aCreationSite)
+    : mCreationSite(aCreationSite)
+    , mMutex("MediaPromise Mutex")
+  {
+    MOZ_COUNT_CTOR(MediaPromise);
+    PROMISE_LOG("%s creating MediaPromise (%p)", mCreationSite, this);
+  }
+
+protected:
+
+  /*
+   * A ThenValue tracks a single consumer waiting on the promise. When a consumer
+   * invokes promise->Then(...), a ThenValue is created. Once the Promise is
+   * resolved or rejected, a ThenValueRunnable is dispatched, which invokes the
+   * resolve/reject method and then deletes the ThenValue.
+   */
+  class ThenValueBase
+  {
+  public:
+    class ThenValueRunnable : public nsRunnable
+    {
+    public:
+      ThenValueRunnable(ThenValueBase* aThenValue, ResolveValueType aResolveValue)
+        : mThenValue(aThenValue)
+      {
+        MOZ_COUNT_CTOR(ThenValueRunnable);
+        mResolveValue.emplace(aResolveValue);
+      }
+
+      ThenValueRunnable(ThenValueBase* aThenValue, RejectValueType aRejectValue)
+        : mThenValue(aThenValue)
+      {
+        MOZ_COUNT_CTOR(ThenValueRunnable);
+        mRejectValue.emplace(aRejectValue);
+      }
+
+      ~ThenValueRunnable()
+      {
+        MOZ_COUNT_DTOR(ThenValueRunnable);
+        MOZ_ASSERT(!mThenValue);
+      }
+
+      NS_IMETHODIMP Run()
+      {
+        PROMISE_LOG("ThenValueRunnable::Run() [this=%p]", this);
+        if (mResolveValue.isSome()) {
+          mThenValue->DoResolve(mResolveValue.ref());
+        } else {
+          mThenValue->DoReject(mRejectValue.ref());
+        }
+
+        delete mThenValue;
+        mThenValue = nullptr;
+        return NS_OK;
+      }
+
+    private:
+      ThenValueBase* mThenValue;
+      Maybe<ResolveValueType> mResolveValue;
+      Maybe<RejectValueType> mRejectValue;
+    };
+
+    ThenValueBase(const char* aCallSite) : mCallSite(aCallSite)
+    {
+      MOZ_COUNT_CTOR(ThenValueBase);
+    }
+
+    virtual void Dispatch(MediaPromise *aPromise) = 0;
+
+  protected:
+    // This may only be deleted by ThenValueRunnable::Run.
+    virtual ~ThenValueBase() { MOZ_COUNT_DTOR(ThenValueBase); }
+
+    virtual void DoResolve(ResolveValueType aResolveValue) = 0;
+    virtual void DoReject(RejectValueType aRejectValue) = 0;
+
+    const char* mCallSite;
+  };
+
+  template<typename TargetType, typename ThisType,
+           typename ResolveMethodType, typename RejectMethodType>
+  class ThenValue : public ThenValueBase
+  {
+  public:
+    ThenValue(TargetType* aResponseTarget, ThisType* aThisVal,
+              ResolveMethodType aResolveMethod, RejectMethodType aRejectMethod,
+              const char* aCallSite)
+      : ThenValueBase(aCallSite)
+      , mResponseTarget(aResponseTarget)
+      , mThisVal(aThisVal)
+      , mResolveMethod(aResolveMethod)
+      , mRejectMethod(aRejectMethod) {}
+
+    void Dispatch(MediaPromise *aPromise) MOZ_OVERRIDE
+    {
+      aPromise->mMutex.AssertCurrentThreadOwns();
+      MOZ_ASSERT(!aPromise->IsPending());
+      bool resolved = aPromise->mResolveValue.isSome();
+      nsRefPtr<nsRunnable> runnable =
+        resolved ? new (typename ThenValueBase::ThenValueRunnable)(this, aPromise->mResolveValue.ref())
+                 : new (typename ThenValueBase::ThenValueRunnable)(this, aPromise->mRejectValue.ref());
+      PROMISE_LOG("%s Then() call made from %s [Runnable=%p, Promise=%p, ThenValue=%p]",
+                  resolved ? "Resolving" : "Rejecting", ThenValueBase::mCallSite,
+                  runnable.get(), aPromise, this);
+      DebugOnly<nsresult> rv = DoDispatch(mResponseTarget, runnable);
+      MOZ_ASSERT(NS_SUCCEEDED(rv));
+    }
+
+  protected:
+    virtual void DoResolve(ResolveValueType aResolveValue)
+    {
+      ((*mThisVal).*mResolveMethod)(aResolveValue);
+    }
+
+    virtual void DoReject(RejectValueType aRejectValue)
+    {
+      ((*mThisVal).*mRejectMethod)(aRejectValue);
+    }
+
+    static nsresult DoDispatch(MediaTaskQueue* aTaskQueue, nsIRunnable* aRunnable)
+    {
+      return aTaskQueue->ForceDispatch(aRunnable);
+    }
+
+    static nsresult DoDispatch(nsIEventTarget* aEventTarget, nsIRunnable* aRunnable)
+    {
+      return aEventTarget->Dispatch(aRunnable, NS_DISPATCH_NORMAL);
+    }
+
+    virtual ~ThenValue() {}
+
+  private:
+    nsRefPtr<TargetType> mResponseTarget;
+    nsRefPtr<ThisType> mThisVal;
+    ResolveMethodType mResolveMethod;
+    RejectMethodType mRejectMethod;
+  };
+public:
+
+  template<typename TargetType, typename ThisType,
+           typename ResolveMethodType, typename RejectMethodType>
+  void Then(TargetType* aResponseTarget, ThisType* aThisVal,
+            ResolveMethodType aResolveMethod, RejectMethodType aRejectMethod,
+            const char* aCallSite)
+  {
+    MutexAutoLock lock(mMutex);
+    ThenValueBase* thenValue = new ThenValue<TargetType, ThisType, ResolveMethodType,
+                                             RejectMethodType>(aResponseTarget, aThisVal,
+                                                               aResolveMethod, aRejectMethod,
+                                                               aCallSite);
+    PROMISE_LOG("%s invoking Then() [this=%p, thenValue=%p, aThisVal=%p, isPending=%d]",
+                aCallSite, this, thenValue, aThisVal, (int) IsPending());
+    if (!IsPending()) {
+      thenValue->Dispatch(this);
+    } else {
+      mThenValues.AppendElement(thenValue);
+    }
+  }
+
+private:
+  // Resolve and Reject may only be invoked via MediaPromiseHolder.
+  friend class MediaPromiseHolder<MediaPromise<ResolveValueType, RejectValueType>>;
+  void Resolve(ResolveValueType aResolveValue, const char* aResolveSite)
+  {
+    MutexAutoLock lock(mMutex);
+    MOZ_ASSERT(IsPending());
+    PROMISE_LOG("%s resolving MediaPromise (%p created at %s)", aResolveSite, this, mCreationSite);
+    mResolveValue.emplace(aResolveValue);
+    DispatchAll();
+  }
+
+  void Reject(RejectValueType aRejectValue, const char* aRejectSite)
+  {
+    MutexAutoLock lock(mMutex);
+    MOZ_ASSERT(IsPending());
+    PROMISE_LOG("%s rejecting MediaPromise (%p created at %s)", aRejectSite, this, mCreationSite);
+    mRejectValue.emplace(aRejectValue);
+    DispatchAll();
+  }
+
+protected:
+  bool IsPending() { return mResolveValue.isNothing() && mRejectValue.isNothing(); }
+  void DispatchAll()
+  {
+    mMutex.AssertCurrentThreadOwns();
+    for (size_t i = 0; i < mThenValues.Length(); ++i)
+      mThenValues[i]->Dispatch(this);
+    mThenValues.Clear();
+  }
+
+  ~MediaPromise()
+  {
+    MOZ_COUNT_DTOR(MediaPromise);
+    PROMISE_LOG("MediaPromise::~MediaPromise [this=%p]", this);
+    MOZ_ASSERT(!IsPending());
+  };
+
+  const char* mCreationSite; // For logging
+  Mutex mMutex;
+  Maybe<ResolveValueType> mResolveValue;
+  Maybe<RejectValueType> mRejectValue;
+  nsTArray<ThenValueBase*> mThenValues;
+};
+
+/*
+ * Class to encapsulate a promise for a particular role. Use this as the member
+ * variable for a class whose method returns a promise.
+ */
+template<typename PromiseType>
+class MediaPromiseHolder
+{
+public:
+  MediaPromiseHolder()
+    : mMonitor(nullptr) {}
+
+  ~MediaPromiseHolder() { MOZ_ASSERT(!mPromise); }
+
+  already_AddRefed<PromiseType> Ensure(const char* aMethodName) {
+    if (mMonitor) {
+      mMonitor->AssertCurrentThreadOwns();
+    }
+    if (!mPromise) {
+      mPromise = new PromiseType(aMethodName);
+    }
+    nsRefPtr<PromiseType> p = mPromise;
+    return p.forget();
+  }
+
+  // Provide a Monitor that should always be held when accessing this instance.
+  void SetMonitor(Monitor* aMonitor) { mMonitor = aMonitor; }
+
+  bool IsEmpty()
+  {
+    if (mMonitor) {
+      mMonitor->AssertCurrentThreadOwns();
+    }
+    return !mPromise;
+  }
+
+  void Resolve(typename PromiseType::ResolveValueType aResolveValue,
+               const char* aMethodName)
+  {
+    if (mMonitor) {
+      mMonitor->AssertCurrentThreadOwns();
+    }
+    MOZ_ASSERT(mPromise);
+    mPromise->Resolve(aResolveValue, aMethodName);
+    mPromise = nullptr;
+  }
+
+  void Reject(typename PromiseType::RejectValueType aRejectValue,
+              const char* aMethodName)
+  {
+    if (mMonitor) {
+      mMonitor->AssertCurrentThreadOwns();
+    }
+    MOZ_ASSERT(mPromise);
+    mPromise->Reject(aRejectValue, aMethodName);
+    mPromise = nullptr;
+  }
+
+private:
+  Monitor* mMonitor;
+  nsRefPtr<PromiseType> mPromise;
+};
+
+#undef PROMISE_LOG
+
+} // namespace mozilla
+
+#endif
diff --git a/dom/media/moz.build b/dom/media/moz.build
index 86ef4410a0fb..76047be095d6 100644
--- a/dom/media/moz.build
+++ b/dom/media/moz.build
@@ -105,6 +105,7 @@ EXPORTS += [
     'MediaDecoderStateMachine.h',
     'MediaInfo.h',
     'MediaMetadataManager.h',
+    'MediaPromise.h',
     'MediaQueue.h',
     'MediaRecorder.h',
     'MediaResource.h',