diff --git a/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImpl.java b/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImpl.java
index 680cbe71ff..a540b18974 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImpl.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImpl.java
@@ -281,7 +281,7 @@ import java.util.concurrent.CopyOnWriteArraySet;
     if (timeline.isEmpty() || pendingSeekAcks > 0) {
       return maskingPeriodIndex;
     } else {
-      return playbackInfo.periodIndex;
+      return playbackInfo.periodId.periodIndex;
     }
   }
 
@@ -290,7 +290,7 @@ import java.util.concurrent.CopyOnWriteArraySet;
     if (timeline.isEmpty() || pendingSeekAcks > 0) {
       return maskingWindowIndex;
     } else {
-      return timeline.getPeriod(playbackInfo.periodIndex, period).windowIndex;
+      return timeline.getPeriod(playbackInfo.periodId.periodIndex, period).windowIndex;
     }
   }
 
@@ -307,7 +307,7 @@ import java.util.concurrent.CopyOnWriteArraySet;
     if (timeline.isEmpty() || pendingSeekAcks > 0) {
       return maskingWindowPositionMs;
     } else {
-      timeline.getPeriod(playbackInfo.periodIndex, period);
+      timeline.getPeriod(playbackInfo.periodId.periodIndex, period);
       return period.getPositionInWindowMs() + C.usToMs(playbackInfo.positionUs);
     }
   }
@@ -318,7 +318,7 @@ import java.util.concurrent.CopyOnWriteArraySet;
     if (timeline.isEmpty() || pendingSeekAcks > 0) {
       return maskingWindowPositionMs;
     } else {
-      timeline.getPeriod(playbackInfo.periodIndex, period);
+      timeline.getPeriod(playbackInfo.periodId.periodIndex, period);
       return period.getPositionInWindowMs() + C.usToMs(playbackInfo.bufferedPositionUs);
     }
   }
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java b/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java
index bd0cf558a7..1a79c50274 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java
@@ -50,21 +50,25 @@ import java.io.IOException;
    */
   public static final class PlaybackInfo {
 
-    public final int periodIndex;
+    public final MediaPeriodId periodId;
     public final long startPositionUs;
 
     public volatile long positionUs;
     public volatile long bufferedPositionUs;
 
     public PlaybackInfo(int periodIndex, long startPositionUs) {
-      this.periodIndex = periodIndex;
+      this(new MediaPeriodId(periodIndex), startPositionUs);
+    }
+
+    public PlaybackInfo(MediaPeriodId periodId, long startPositionUs) {
+      this.periodId = periodId;
       this.startPositionUs = startPositionUs;
       positionUs = startPositionUs;
       bufferedPositionUs = startPositionUs;
     }
 
-    public PlaybackInfo copyWithPeriodIndex(int periodIndex) {
-      PlaybackInfo playbackInfo = new PlaybackInfo(periodIndex, startPositionUs);
+    public PlaybackInfo copyWithPeriodId(MediaPeriodId periodId) {
+      PlaybackInfo playbackInfo = new PlaybackInfo(periodId.periodIndex, startPositionUs);
       playbackInfo.positionUs = positionUs;
       playbackInfo.bufferedPositionUs = bufferedPositionUs;
       return playbackInfo;
@@ -654,7 +658,7 @@ import java.io.IOException;
     long periodPositionUs = periodPosition.second;
 
     try {
-      if (periodIndex == playbackInfo.periodIndex
+      if (periodIndex == playbackInfo.periodId.periodIndex
           && ((periodPositionUs / 1000) == (playbackInfo.positionUs / 1000))) {
         // Seek position equals the current position. Do nothing.
         return;
@@ -1017,8 +1021,8 @@ import java.io.IOException;
 
     // The current period is in the new timeline. Update the holder and playbackInfo.
     periodHolder.setPeriodIndex(periodIndex, isFinalPeriod(periodIndex));
-    if (periodIndex != playbackInfo.periodIndex) {
-      playbackInfo = playbackInfo.copyWithPeriodIndex(periodIndex);
+    if (periodIndex != playbackInfo.periodId.periodIndex) {
+      playbackInfo = playbackInfo.copyWithPeriodId(new MediaPeriodId(periodIndex));
     }
 
     // If there are subsequent holders, update the index for each of them. If we find a holder
@@ -1301,7 +1305,7 @@ import java.io.IOException;
   private void maybeUpdateLoadingPeriod() throws IOException {
     int newLoadingPeriodIndex;
     if (loadingPeriodHolder == null) {
-      newLoadingPeriodIndex = playbackInfo.periodIndex;
+      newLoadingPeriodIndex = playbackInfo.periodId.periodIndex;
     } else {
       int loadingPeriodIndex = loadingPeriodHolder.periodIndex;
       if (loadingPeriodHolder.isFinal || !loadingPeriodHolder.isFullyBuffered()
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/MediaSource.java b/library/core/src/main/java/com/google/android/exoplayer2/source/MediaSource.java
index d696b43dd7..77e3b6be65 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/MediaSource.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/MediaSource.java
@@ -16,6 +16,7 @@
 package com.google.android.exoplayer2.source;
 
 import android.support.annotation.Nullable;
+import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.ExoPlayer;
 import com.google.android.exoplayer2.Timeline;
 import com.google.android.exoplayer2.upstream.Allocator;
@@ -51,13 +52,28 @@ public interface MediaSource {
      */
     public final int periodIndex;
 
+    /**
+     * If the media period is in an ad break, the index of the ad break in the period.
+     * {@link C#INDEX_UNSET} otherwise.
+     */
+    public final int adBreakIndex;
+
+    /**
+     * If the media period is in an ad break, the index of the ad in its ad break in the period.
+     * {@link C#INDEX_UNSET} otherwise.
+     */
+    public final int adIndexInAdBreak;
+
     /**
      * Creates a media period identifier for the specified period in the timeline.
      *
      * @param periodIndex The timeline period index.
      */
     public MediaPeriodId(int periodIndex) {
+      // TODO: Allow creation of MediaPeriodIds for ad breaks.
       this.periodIndex = periodIndex;
+      adBreakIndex = C.INDEX_UNSET;
+      adIndexInAdBreak = C.INDEX_UNSET;
     }
 
   }