diff --git a/packager/media/base/buffer_reader.cc b/packager/media/base/buffer_reader.cc
index 7189ea1a66..4e593550af 100644
--- a/packager/media/base/buffer_reader.cc
+++ b/packager/media/base/buffer_reader.cc
@@ -53,6 +53,15 @@ bool BufferReader::ReadToVector(std::vector<uint8_t>* vec, size_t count) {
   return true;
 }
 
+bool BufferReader::ReadToString(std::string* str, size_t size) {
+  DCHECK(str);
+  if (!HasBytes(size))
+    return false;
+  str->assign(buf_ + pos_, buf_ + pos_ + size);
+  pos_ += size;
+  return true;
+}
+
 bool BufferReader::SkipBytes(size_t num_bytes) {
   if (!HasBytes(num_bytes))
     return false;
diff --git a/packager/media/base/buffer_reader.h b/packager/media/base/buffer_reader.h
index 2d85a7764f..46b615780a 100644
--- a/packager/media/base/buffer_reader.h
+++ b/packager/media/base/buffer_reader.h
@@ -9,6 +9,7 @@
 
 #include <stdint.h>
 
+#include <string>
 #include <vector>
 
 #include "packager/base/compiler_specific.h"
@@ -53,6 +54,7 @@ class BufferReader {
   /// @}
 
   bool ReadToVector(std::vector<uint8_t>* t, size_t count) WARN_UNUSED_RESULT;
+  bool ReadToString(std::string* str, size_t size) WARN_UNUSED_RESULT;
 
   /// Advance the stream by this many bytes.
   /// @return false if there are not enough bytes in the buffer, true otherwise.
diff --git a/packager/media/formats/mp4/box_buffer.h b/packager/media/formats/mp4/box_buffer.h
index 4a20b498bb..349b1b592e 100644
--- a/packager/media/formats/mp4/box_buffer.h
+++ b/packager/media/formats/mp4/box_buffer.h
@@ -7,6 +7,8 @@
 #ifndef MEDIA_FORMATS_MP4_BOX_BUFFER_H_
 #define MEDIA_FORMATS_MP4_BOX_BUFFER_H_
 
+#include <string>
+
 #include "packager/base/compiler_specific.h"
 #include "packager/media/base/buffer_writer.h"
 #include "packager/media/formats/mp4/box.h"
@@ -55,6 +57,14 @@ class BoxBuffer {
     return writer_->Size();
   }
 
+  /// @return In read mode, return the number of bytes left in the box.
+  ///         In write mode, return 0.
+  size_t BytesLeft() const {
+    if (reader_)
+      return reader_->size() - reader_->pos();
+    return 0;
+  }
+
   /// @name Read/write integers of various sizes and signedness.
   /// @{
   bool ReadWriteUInt8(uint8_t* v) {
@@ -120,9 +130,21 @@ class BoxBuffer {
     if (reader_)
       return reader_->ReadToVector(vector, count);
     DCHECK_EQ(vector->size(), count);
-    writer_->AppendVector(*vector);
+    writer_->AppendArray(vector_as_array(vector), count);
     return true;
   }
+
+  /// Reads @a size characters from the buffer and sets it to str.
+  /// Writes @a str to the buffer. Write mode ignores @a size.
+  bool ReadWriteString(std::string* str, size_t size) {
+    if (reader_)
+      return reader_->ReadToString(str, size);
+    DCHECK_EQ(str->size(), size);
+    writer_->AppendArray(reinterpret_cast<const uint8_t*>(str->data()),
+                         str->size());
+    return true;
+  }
+
   bool ReadWriteFourCC(FourCC* fourcc) {
     if (reader_)
       return reader_->ReadFourCC(fourcc);
diff --git a/packager/media/formats/mp4/box_definitions.cc b/packager/media/formats/mp4/box_definitions.cc
index 13a18b4f2b..d13294c023 100644
--- a/packager/media/formats/mp4/box_definitions.cc
+++ b/packager/media/formats/mp4/box_definitions.cc
@@ -25,6 +25,7 @@ const uint8_t kUnityMatrix[] = {0, 1, 0, 0, 0, 0, 0, 0, 0,    0, 0, 0,
 // Default entries for HandlerReference box.
 const char kVideoHandlerName[] = "VideoHandler";
 const char kAudioHandlerName[] = "SoundHandler";
+const char kTextHandlerName[] = "TextHandler";
 
 // Default values for VideoSampleEntry box.
 const uint32_t kVideoResolution = 0x00480000;  // 72 dpi.
@@ -36,6 +37,10 @@ const char kAvcCompressorName[] = "\012AVC Coding";
 const char kHevcCompressorName[] = "\013HEVC Coding";
 const char kVpcCompressorName[] = "\012VPC Coding";
 
+// Using negative value as "not set". It is very unlikely that 2^31 cues happen
+// at once.
+const int kCueSourceIdNotSet = -1;
+
 // Utility functions to check if the 64bit integers can fit in 32bit integer.
 bool IsFitIn32Bits(uint64_t a) {
   return a <= std::numeric_limits<uint32_t>::max();
@@ -377,10 +382,20 @@ FourCC SampleDescription::BoxType() const { return FOURCC_STSD; }
 
 bool SampleDescription::ReadWriteInternal(BoxBuffer* buffer) {
   uint32_t count = 0;
-  if (type == kVideo)
-    count = video_entries.size();
-  else
-    count = audio_entries.size();
+  switch (type) {
+    case kVideo:
+      count = video_entries.size();
+      break;
+    case kAudio:
+      count = audio_entries.size();
+      break;
+    case kText:
+      count = wvtt_entries.size();
+      break;
+    default:
+      NOTIMPLEMENTED() << "SampleDecryption type " << type
+                       << " is not handled. Skipping.";
+  }
   RCHECK(ReadWriteHeaderInternal(buffer) &&
          buffer->ReadWriteUInt32(&count));
 
@@ -397,6 +412,9 @@ bool SampleDescription::ReadWriteInternal(BoxBuffer* buffer) {
     } else if (type == kAudio) {
       RCHECK(reader->ReadAllChildren(&audio_entries));
       RCHECK(audio_entries.size() == count);
+    } else if (type == kText) {
+      RCHECK(reader->ReadAllChildren(&wvtt_entries));
+      RCHECK(wvtt_entries.size() == count);
     }
   } else {
     DCHECK_LT(0u, count);
@@ -406,6 +424,9 @@ bool SampleDescription::ReadWriteInternal(BoxBuffer* buffer) {
     } else if (type == kAudio) {
       for (uint32_t i = 0; i < count; ++i)
         RCHECK(buffer->ReadWriteChild(&audio_entries[i]));
+    } else if (type == kText) {
+      for (uint32_t i = 0; i < count; ++i)
+        RCHECK(buffer->ReadWriteChild(&wvtt_entries[i]));
     } else {
       NOTIMPLEMENTED();
     }
@@ -803,6 +824,10 @@ bool HandlerReference::ReadWriteInternal(BoxBuffer* buffer) {
       hdlr_type = FOURCC_SOUN;
       handler_name.assign(kAudioHandlerName,
                           kAudioHandlerName + arraysize(kAudioHandlerName));
+    } else if (type == kText) {
+      hdlr_type = FOURCC_TEXT;
+      handler_name.assign(kTextHandlerName,
+                          kTextHandlerName + arraysize(kTextHandlerName));
     } else {
       NOTIMPLEMENTED();
       return false;
@@ -828,9 +853,15 @@ bool HandlerReference::ReadWriteInternal(BoxBuffer* buffer) {
 }
 
 uint32_t HandlerReference::ComputeSizeInternal() {
-  return HeaderSize() + kFourCCSize + 16 +  // 16 bytes Reserved
-         (type == kVideo ? sizeof(kVideoHandlerName)
-                         : sizeof(kAudioHandlerName));
+  uint32_t box_size = HeaderSize() + kFourCCSize + 16;  // 16 bytes Reserved
+  if (type == kVideo) {
+    box_size += sizeof(kVideoHandlerName);
+  } else if (type == kAudio) {
+    box_size += sizeof(kAudioHandlerName);
+  } else {
+    box_size += sizeof(kTextHandlerName);
+  }
+  return box_size;
 }
 
 CodecConfigurationRecord::CodecConfigurationRecord() : box_type(FOURCC_NULL) {}
@@ -844,7 +875,7 @@ FourCC CodecConfigurationRecord::BoxType() const {
 bool CodecConfigurationRecord::ReadWriteInternal(BoxBuffer* buffer) {
   RCHECK(ReadWriteHeaderInternal(buffer));
   if (buffer->Reading()) {
-    RCHECK(buffer->ReadWriteVector(&data, buffer->Size() - buffer->Pos()));
+    RCHECK(buffer->ReadWriteVector(&data, buffer->BytesLeft()));
   } else {
     RCHECK(buffer->ReadWriteVector(&data, data.size()));
   }
@@ -996,7 +1027,7 @@ bool ElementaryStreamDescriptor::ReadWriteInternal(BoxBuffer* buffer) {
   RCHECK(ReadWriteHeaderInternal(buffer));
   if (buffer->Reading()) {
     std::vector<uint8_t> data;
-    RCHECK(buffer->ReadWriteVector(&data, buffer->Size() - buffer->Pos()));
+    RCHECK(buffer->ReadWriteVector(&data, buffer->BytesLeft()));
     RCHECK(es_descriptor.Parse(data));
     if (es_descriptor.IsAAC()) {
       RCHECK(aac_audio_specific_config.Parse(
@@ -1104,6 +1135,68 @@ uint32_t AudioSampleEntry::ComputeSizeInternal() {
          4;   // 4 bytes predefined.
 }
 
+WebVTTConfigurationBox::WebVTTConfigurationBox() {}
+WebVTTConfigurationBox::~WebVTTConfigurationBox() {}
+
+FourCC WebVTTConfigurationBox::BoxType() const {
+  return FOURCC_vttC;
+}
+
+bool WebVTTConfigurationBox::ReadWriteInternal(BoxBuffer* buffer) {
+  RCHECK(ReadWriteHeaderInternal(buffer));
+  return buffer->ReadWriteString(
+      &config,
+      buffer->Reading() ? buffer->BytesLeft() : config.size());
+}
+
+uint32_t WebVTTConfigurationBox::ComputeSizeInternal() {
+  return HeaderSize() + config.size();
+}
+
+WebVTTSourceLabelBox::WebVTTSourceLabelBox() {}
+WebVTTSourceLabelBox::~WebVTTSourceLabelBox() {}
+
+FourCC WebVTTSourceLabelBox::BoxType() const {
+  return FOURCC_vlab;
+}
+
+bool WebVTTSourceLabelBox::ReadWriteInternal(BoxBuffer* buffer) {
+  RCHECK(ReadWriteHeaderInternal(buffer));
+  return buffer->ReadWriteString(&source_label, buffer->Reading()
+                                                    ? buffer->BytesLeft()
+                                                    : source_label.size());
+}
+
+uint32_t WebVTTSourceLabelBox::ComputeSizeInternal() {
+  if (source_label.empty())
+    return 0;
+  return HeaderSize() + source_label.size();
+}
+
+WVTTSampleEntry::WVTTSampleEntry() {}
+WVTTSampleEntry::~WVTTSampleEntry() {}
+
+FourCC WVTTSampleEntry::BoxType() const {
+  return FOURCC_wvtt;
+}
+
+bool WVTTSampleEntry::ReadWriteInternal(BoxBuffer* buffer) {
+  // TODO(rkuroiwa): Handle the optional MPEG4BitRateBox.
+  RCHECK(ReadWriteHeaderInternal(buffer) &&
+         buffer->IgnoreBytes(6) &&  // reserved for SampleEntry.
+         buffer->ReadWriteUInt16(&data_reference_index) &&
+         buffer->PrepareChildren() &&
+         buffer->ReadWriteChild(&config) &&
+         buffer->ReadWriteChild(&label));
+  return true;
+}
+
+uint32_t WVTTSampleEntry::ComputeSizeInternal() {
+  // 6 for the (anonymous) reserved bytes for SampleEntry class.
+  return HeaderSize() + 6 + sizeof(data_reference_index) +
+         config.ComputeSize() + label.ComputeSize();
+}
+
 MediaHeader::MediaHeader()
     : creation_time(0), modification_time(0), timescale(0), duration(0) {
   language[0] = 0;
@@ -1192,6 +1285,19 @@ uint32_t SoundMediaHeader::ComputeSizeInternal() {
   return HeaderSize() + sizeof(balance) + sizeof(uint16_t);
 }
 
+SubtitleMediaHeader::SubtitleMediaHeader() {}
+SubtitleMediaHeader::~SubtitleMediaHeader() {}
+
+FourCC SubtitleMediaHeader::BoxType() const { return FOURCC_sthd; }
+
+bool SubtitleMediaHeader::ReadWriteInternal(BoxBuffer* buffer) {
+  return ReadWriteHeaderInternal(buffer);
+}
+
+uint32_t SubtitleMediaHeader::ComputeSizeInternal() {
+  return HeaderSize();
+}
+
 DataEntryUrl::DataEntryUrl() {
   const uint32_t kDataEntryUrlFlags = 1;
   flags = kDataEntryUrlFlags;
@@ -1260,12 +1366,19 @@ bool MediaInformation::ReadWriteInternal(BoxBuffer* buffer) {
          buffer->PrepareChildren() &&
          buffer->ReadWriteChild(&dinf) &&
          buffer->ReadWriteChild(&sample_table));
-  if (sample_table.description.type == kVideo)
-    RCHECK(buffer->ReadWriteChild(&vmhd));
-  else if (sample_table.description.type == kAudio)
-    RCHECK(buffer->ReadWriteChild(&smhd));
-  else
-    NOTIMPLEMENTED();
+  switch (sample_table.description.type) {
+    case kVideo:
+      RCHECK(buffer->ReadWriteChild(&vmhd));
+      break;
+    case kAudio:
+      RCHECK(buffer->ReadWriteChild(&smhd));
+      break;
+    case kText:
+      RCHECK(buffer->ReadWriteChild(&sthd));
+      break;
+    default:
+      NOTIMPLEMENTED();
+  }
   // Hint is not supported for now.
   return true;
 }
@@ -1273,10 +1386,19 @@ bool MediaInformation::ReadWriteInternal(BoxBuffer* buffer) {
 uint32_t MediaInformation::ComputeSizeInternal() {
   uint32_t box_size =
       HeaderSize() + dinf.ComputeSize() + sample_table.ComputeSize();
-  if (sample_table.description.type == kVideo)
-    box_size += vmhd.ComputeSize();
-  else if (sample_table.description.type == kAudio)
-    box_size += smhd.ComputeSize();
+  switch (sample_table.description.type) {
+    case kVideo:
+      box_size += vmhd.ComputeSize();
+      break;
+    case kAudio:
+      box_size += smhd.ComputeSize();
+      break;
+    case kText:
+      box_size += sthd.ComputeSize();
+      break;
+    default:
+      NOTIMPLEMENTED();
+  }
   return box_size;
 }
 
@@ -1912,6 +2034,154 @@ uint32_t MediaData::ComputeSizeInternal() {
   return HeaderSize() + data_size;
 }
 
+CueSourceIDBox::CueSourceIDBox() : source_id(kCueSourceIdNotSet) {}
+CueSourceIDBox::~CueSourceIDBox() {}
+
+FourCC CueSourceIDBox::BoxType() const { return FOURCC_vsid; }
+
+bool CueSourceIDBox::ReadWriteInternal(BoxBuffer* buffer) {
+  RCHECK(ReadWriteHeaderInternal(buffer) && buffer->ReadWriteInt32(&source_id));
+  return true;
+}
+
+uint32_t CueSourceIDBox::ComputeSizeInternal() {
+  if (source_id == kCueSourceIdNotSet)
+    return 0;
+  return HeaderSize() + sizeof(source_id);
+}
+
+CueTimeBox::CueTimeBox() {}
+CueTimeBox::~CueTimeBox() {}
+
+FourCC CueTimeBox::BoxType() const {
+  return FOURCC_ctim;
+}
+
+bool CueTimeBox::ReadWriteInternal(BoxBuffer* buffer) {
+  RCHECK(ReadWriteHeaderInternal(buffer));
+  return buffer->ReadWriteString(
+      &cue_current_time,
+      buffer->Reading() ? buffer->BytesLeft() : cue_current_time.size());
+}
+
+uint32_t CueTimeBox::ComputeSizeInternal() {
+  if (cue_current_time.empty())
+    return 0;
+  return HeaderSize() + cue_current_time.size();
+}
+
+CueIDBox::CueIDBox() {}
+CueIDBox::~CueIDBox() {}
+
+FourCC CueIDBox::BoxType() const {
+  return FOURCC_iden;
+}
+
+bool CueIDBox::ReadWriteInternal(BoxBuffer* buffer) {
+  RCHECK(ReadWriteHeaderInternal(buffer));
+  return buffer->ReadWriteString(
+      &cue_id, buffer->Reading() ? buffer->BytesLeft() : cue_id.size());
+}
+
+uint32_t CueIDBox::ComputeSizeInternal() {
+  if (cue_id.empty())
+    return 0;
+  return HeaderSize() + cue_id.size();
+}
+
+CueSettingsBox::CueSettingsBox() {}
+CueSettingsBox::~CueSettingsBox() {}
+
+FourCC CueSettingsBox::BoxType() const {
+  return FOURCC_sttg;
+}
+
+bool CueSettingsBox::ReadWriteInternal(BoxBuffer* buffer) {
+  RCHECK(ReadWriteHeaderInternal(buffer));
+  return buffer->ReadWriteString(
+      &settings, buffer->Reading() ? buffer->BytesLeft() : settings.size());
+}
+
+uint32_t CueSettingsBox::ComputeSizeInternal() {
+  if (settings.empty())
+    return 0;
+  return HeaderSize() + settings.size();
+}
+
+CuePayloadBox::CuePayloadBox() {}
+CuePayloadBox::~CuePayloadBox() {}
+
+FourCC CuePayloadBox::BoxType() const {
+  return FOURCC_payl;
+}
+
+bool CuePayloadBox::ReadWriteInternal(BoxBuffer* buffer) {
+  RCHECK(ReadWriteHeaderInternal(buffer));
+  return buffer->ReadWriteString(
+      &cue_text, buffer->Reading() ? buffer->BytesLeft() : cue_text.size());
+}
+
+uint32_t CuePayloadBox::ComputeSizeInternal() {
+  return HeaderSize() + cue_text.size();
+}
+
+VTTEmptyCueBox::VTTEmptyCueBox() {}
+VTTEmptyCueBox::~VTTEmptyCueBox() {}
+
+FourCC VTTEmptyCueBox::BoxType() const {
+  return FOURCC_vtte;
+}
+
+bool VTTEmptyCueBox::ReadWriteInternal(BoxBuffer* buffer) {
+  return ReadWriteHeaderInternal(buffer);
+}
+
+uint32_t VTTEmptyCueBox::ComputeSizeInternal() {
+  return HeaderSize();
+}
+
+VTTAdditionalTextBox::VTTAdditionalTextBox() {}
+VTTAdditionalTextBox::~VTTAdditionalTextBox() {}
+
+FourCC VTTAdditionalTextBox::BoxType() const {
+  return FOURCC_vtta;
+}
+
+bool VTTAdditionalTextBox::ReadWriteInternal(BoxBuffer* buffer) {
+  RCHECK(ReadWriteHeaderInternal(buffer));
+  return buffer->ReadWriteString(
+      &cue_additional_text,
+      buffer->Reading() ? buffer->BytesLeft() : cue_additional_text.size());
+}
+
+uint32_t VTTAdditionalTextBox::ComputeSizeInternal() {
+  return HeaderSize() + cue_additional_text.size();
+}
+
+VTTCueBox::VTTCueBox() {}
+VTTCueBox::~VTTCueBox() {}
+
+FourCC VTTCueBox::BoxType() const  {
+  return FOURCC_vttc;
+}
+
+bool VTTCueBox::ReadWriteInternal(BoxBuffer* buffer) {
+  RCHECK(ReadWriteHeaderInternal(buffer) &&
+         buffer->PrepareChildren() &&
+         buffer->ReadWriteChild(&cue_source_id) &&
+         buffer->ReadWriteChild(&cue_id) &&
+         buffer->ReadWriteChild(&cue_time) &&
+         buffer->ReadWriteChild(&cue_settings) &&
+         buffer->ReadWriteChild(&cue_payload));
+  return true;
+}
+
+uint32_t VTTCueBox::ComputeSizeInternal() {
+  return HeaderSize() + cue_source_id.ComputeSize() + cue_id.ComputeSize() +
+         cue_time.ComputeSize() + cue_settings.ComputeSize() +
+         cue_payload.ComputeSize();
+}
+
 }  // namespace mp4
 }  // namespace media
 }  // namespace edash_packager
diff --git a/packager/media/formats/mp4/box_definitions.h b/packager/media/formats/mp4/box_definitions.h
index ffabc5404d..cb4f58be24 100644
--- a/packager/media/formats/mp4/box_definitions.h
+++ b/packager/media/formats/mp4/box_definitions.h
@@ -23,7 +23,8 @@ enum TrackType {
   kInvalid = 0,
   kVideo,
   kAudio,
-  kHint
+  kHint,
+  kText,
 };
 
 class BoxBuffer;
@@ -233,12 +234,33 @@ struct AudioSampleEntry : Box {
   DTSSpecific ddts;
 };
 
+struct WebVTTConfigurationBox : Box {
+  DECLARE_BOX_METHODS(WebVTTConfigurationBox);
+  std::string config;
+};
+
+struct WebVTTSourceLabelBox : Box {
+  DECLARE_BOX_METHODS(WebVTTSourceLabelBox);
+  std::string source_label;
+};
+
+struct WVTTSampleEntry : Box {
+  DECLARE_BOX_METHODS(WVTTSampleEntry);
+
+  uint16_t data_reference_index;
+
+  WebVTTConfigurationBox config;
+  WebVTTSourceLabelBox label;
+  // Optional MPEG4BitRateBox.
+};
+
 struct SampleDescription : FullBox {
   DECLARE_BOX_METHODS(SampleDescription);
 
   TrackType type;
   std::vector<VideoSampleEntry> video_entries;
   std::vector<AudioSampleEntry> audio_entries;
+  std::vector<WVTTSampleEntry> wvtt_entries;
 };
 
 struct DecodingTime {
@@ -358,6 +380,10 @@ struct SoundMediaHeader : FullBox {
   uint16_t balance;
 };
 
+struct SubtitleMediaHeader : FullBox {
+  DECLARE_BOX_METHODS(SubtitleMediaHeader);
+};
+
 struct DataEntryUrl : FullBox {
   DECLARE_BOX_METHODS(DataEntryUrl);
 
@@ -385,6 +411,7 @@ struct MediaInformation : Box {
   // Exactly one specific meida header shall be present, vmhd, smhd, hmhd, nmhd.
   VideoMediaHeader vmhd;
   SoundMediaHeader smhd;
+  SubtitleMediaHeader sthd;
 };
 
 struct Media : Box {
@@ -591,6 +618,50 @@ struct MediaData : Box {
   uint32_t data_size;
 };
 
+struct CueSourceIDBox : Box {
+  DECLARE_BOX_METHODS(CueSourceIDBox);
+  int32_t source_id;
+};
+
+struct CueTimeBox : Box {
+  DECLARE_BOX_METHODS(CueTimeBox);
+  std::string cue_current_time;
+};
+
+struct CueIDBox : Box {
+  DECLARE_BOX_METHODS(CueIDBox);
+  std::string cue_id;
+};
+
+struct CueSettingsBox : Box {
+  DECLARE_BOX_METHODS(CueSettingsBox);
+  std::string settings;
+};
+
+struct CuePayloadBox : Box {
+  DECLARE_BOX_METHODS(CuePayloadBox);
+  std::string cue_text;
+};
+
+struct VTTEmptyCueBox : Box {
+  DECLARE_BOX_METHODS(VTTEmptyCueBox);
+};
+
+struct VTTAdditionalTextBox : Box {
+  DECLARE_BOX_METHODS(VTTAdditionalTextBox);
+  std::string cue_additional_text;
+};
+
+struct VTTCueBox : Box {
+  DECLARE_BOX_METHODS(VTTCueBox);
+
+  CueSourceIDBox cue_source_id;
+  CueIDBox cue_id;
+  CueTimeBox cue_time;
+  CueSettingsBox cue_settings;
+  CuePayloadBox cue_payload;
+};
+
 #undef DECLARE_BOX
 
 }  // namespace mp4
diff --git a/packager/media/formats/mp4/box_definitions_comparison.h b/packager/media/formats/mp4/box_definitions_comparison.h
index 6788731f05..4949b38061 100644
--- a/packager/media/formats/mp4/box_definitions_comparison.h
+++ b/packager/media/formats/mp4/box_definitions_comparison.h
@@ -209,6 +209,21 @@ inline bool operator==(const AudioSampleEntry& lhs,
          lhs.ddts == rhs.ddts;
 }
 
+inline bool operator==(const WebVTTConfigurationBox& lhs,
+                       const WebVTTConfigurationBox& rhs) {
+  return lhs.config == rhs.config;
+}
+
+inline bool operator==(const WebVTTSourceLabelBox& lhs,
+                       const WebVTTSourceLabelBox& rhs) {
+  return lhs.source_label == rhs.source_label;
+}
+
+inline bool operator==(const WVTTSampleEntry& lhs,
+                       const WVTTSampleEntry& rhs) {
+  return lhs.config == rhs.config && lhs.label == rhs.label;
+}
+
 inline bool operator==(const MediaHeader& lhs, const MediaHeader& rhs) {
   return lhs.creation_time == rhs.creation_time &&
          lhs.modification_time == rhs.modification_time &&
@@ -229,6 +244,11 @@ inline bool operator==(const SoundMediaHeader& lhs,
   return lhs.balance == rhs.balance;
 }
 
+inline bool operator==(const SubtitleMediaHeader& lhs,
+                       const SubtitleMediaHeader& rhs) {
+  return true;
+}
+
 inline bool operator==(const DataEntryUrl& lhs, const DataEntryUrl& rhs) {
   return lhs.flags == rhs.flags && lhs.location == rhs.location;
 }
@@ -366,6 +386,47 @@ inline bool operator==(const SegmentIndex& lhs, const SegmentIndex& rhs) {
          lhs.references == rhs.references;
 }
 
+inline bool operator==(const CueSourceIDBox& lhs,
+                       const CueSourceIDBox& rhs) {
+  return lhs.source_id == rhs.source_id;
+}
+
+inline bool operator==(const CueTimeBox& lhs,
+                       const CueTimeBox& rhs) {
+  return lhs.cue_current_time == rhs.cue_current_time;
+}
+
+inline bool operator==(const CueIDBox& lhs,
+                       const CueIDBox& rhs) {
+  return lhs.cue_id == rhs.cue_id;
+}
+
+inline bool operator==(const CueSettingsBox& lhs,
+                       const CueSettingsBox& rhs) {
+  return lhs.settings == rhs.settings;
+}
+
+inline bool operator==(const CuePayloadBox& lhs,
+                       const CuePayloadBox& rhs) {
+  return lhs.cue_text == rhs.cue_text;
+}
+
+inline bool operator==(const VTTEmptyCueBox& lhs, const VTTEmptyCueBox& rhs) {
+  return true;
+}
+
+inline bool operator==(const VTTAdditionalTextBox& lhs,
+                       const VTTAdditionalTextBox& rhs) {
+  return lhs.cue_additional_text == rhs.cue_additional_text;
+}
+
+inline bool operator==(const VTTCueBox& lhs,
+                       const VTTCueBox& rhs) {
+  return lhs.cue_source_id == rhs.cue_source_id && lhs.cue_id == rhs.cue_id &&
+         lhs.cue_time == rhs.cue_time && lhs.cue_settings == rhs.cue_settings &&
+         lhs.cue_payload == rhs.cue_payload;
+}
+
 }  // namespace mp4
 }  // namespace media
 }  // namespace edash_packager
diff --git a/packager/media/formats/mp4/box_definitions_unittest.cc b/packager/media/formats/mp4/box_definitions_unittest.cc
index 0f6239d048..dc0fdc036a 100644
--- a/packager/media/formats/mp4/box_definitions_unittest.cc
+++ b/packager/media/formats/mp4/box_definitions_unittest.cc
@@ -335,6 +335,33 @@ class BoxDefinitionsTestGeneral : public testing::Test {
 
   void Modify(AudioSampleEntry* enca) { enca->channelcount = 2; }
 
+  void Fill(WebVTTConfigurationBox* vttc) {
+    vttc->config = "WEBVTT";
+  }
+
+  void Modify(WebVTTConfigurationBox* vttc) {
+    vttc->config = "WEBVTT\n"
+                   "Region: id=someting width=40\% lines=3";
+  }
+
+  void Fill(WebVTTSourceLabelBox* vlab) {
+    vlab->source_label = "some_label";
+  }
+
+  void Modify(WebVTTSourceLabelBox* vlab) {
+    vlab->source_label = "another_label";
+  }
+
+  void Fill(WVTTSampleEntry* wvtt) {
+    Fill(&wvtt->config);
+    Fill(&wvtt->label);
+  }
+
+  void Modify(WVTTSampleEntry* wvtt) {
+    Modify(&wvtt->config);
+    Modify(&wvtt->label);
+  }
+
   void Fill(SampleDescription* stsd) {
     stsd->type = kSampleDescriptionTrackType;
     stsd->video_entries.resize(1);
@@ -475,6 +502,9 @@ class BoxDefinitionsTestGeneral : public testing::Test {
 
   void Modify(SoundMediaHeader* smhd) { smhd->balance /= 2; }
 
+  void Fill(SubtitleMediaHeader* sthd) {}
+  void Modify(SubtitleMediaHeader* sthd) {}
+
   void Fill(DataEntryUrl* url) {
     url->flags = 2;
     url->location.assign(kData8, kData8 + arraysize(kData8));
@@ -718,6 +748,73 @@ class BoxDefinitionsTestGeneral : public testing::Test {
     sidx->version = 1;
   }
 
+  void Fill(CueSourceIDBox* vsid) {
+    vsid->source_id = 5;
+  }
+
+  void Modify(CueSourceIDBox* vsid) {
+    vsid->source_id = 100;
+  }
+
+  void Fill(CueTimeBox* ctim) {
+    ctim->cue_current_time = "00:19:00.000";
+  }
+
+  void Modify(CueTimeBox* ctim) {
+    ctim->cue_current_time = "00:20:01.291";
+  }
+
+  void Fill(CueIDBox* iden) {
+    iden->cue_id = "some_id";
+  }
+
+  void Modify(CueIDBox* iden) {
+    iden->cue_id = "another_id";
+  }
+
+  void Fill(CueSettingsBox* sttg) {
+    sttg->settings = "align:left";
+  }
+
+  void Modify(CueSettingsBox* sttg) {
+    sttg->settings = "align:right";
+  }
+
+  void Fill(CuePayloadBox* payl) {
+    payl->cue_text = "hello";
+  }
+
+  void Modify(CuePayloadBox* payl) {
+    payl->cue_text = "hi";
+  }
+
+  void Fill(VTTEmptyCueBox* vtte) {}
+  void Modify(VTTEmptyCueBox* vtte) {}
+
+  void Fill(VTTAdditionalTextBox* vtta) {
+    vtta->cue_additional_text = "NOTE some comment";
+  }
+
+  void Modify(VTTAdditionalTextBox* vtta) {
+    vtta->cue_additional_text = "NOTE another comment";
+  }
+
+  void Fill(VTTCueBox* vttc) {
+    Fill(&vttc->cue_source_id);
+    Fill(&vttc->cue_id);
+    Fill(&vttc->cue_time);
+    Fill(&vttc->cue_settings);
+    Fill(&vttc->cue_payload);
+  }
+
+  void Modify(VTTCueBox* vttc) {
+    Modify(&vttc->cue_source_id);
+    Modify(&vttc->cue_id);
+    Modify(&vttc->cue_time);
+    Modify(&vttc->cue_settings);
+    Modify(&vttc->cue_payload);
+  }
+
   bool IsOptional(const SampleAuxiliaryInformationOffset* box) { return true; }
   bool IsOptional(const SampleAuxiliaryInformationSize* box) { return true; }
   bool IsOptional(const ProtectionSchemeInfo* box) { return true; }
@@ -726,12 +823,18 @@ class BoxDefinitionsTestGeneral : public testing::Test {
   bool IsOptional(const CodecConfigurationRecord* box) { return true; }
   bool IsOptional(const PixelAspectRatio* box) { return true; }
   bool IsOptional(const ElementaryStreamDescriptor* box) { return true; }
+  // Recommended, but optional.
+  bool IsOptional(const WebVTTSourceLabelBox* box) { return true; }
   bool IsOptional(const CompositionTimeToSample* box) { return true; }
   bool IsOptional(const SyncSample* box) { return true; }
   bool IsOptional(const MovieExtendsHeader* box) { return true; }
   bool IsOptional(const MovieExtends* box) { return true; }
   bool IsOptional(const SampleToGroup* box) { return true; }
   bool IsOptional(const SampleGroupDescription* box) { return true; }
+  bool IsOptional(const CueSourceIDBox* box) { return true; }
+  bool IsOptional(const CueIDBox* box) { return true; }
+  bool IsOptional(const CueTimeBox* box) { return true; }
+  bool IsOptional(const CueSettingsBox* box) { return true; }
 
  protected:
   scoped_ptr<BufferWriter> buffer_;
@@ -758,6 +861,9 @@ typedef testing::Types<
     VideoSampleEntry,
     ElementaryStreamDescriptor,
     AudioSampleEntry,
+    WebVTTConfigurationBox,
+    WebVTTSourceLabelBox,
+    WVTTSampleEntry,
     SampleDescription,
     DecodingTimeToSample,
     CompositionTimeToSample,
@@ -771,6 +877,7 @@ typedef testing::Types<
     MediaHeader,
     VideoMediaHeader,
     SoundMediaHeader,
+    SubtitleMediaHeader,
     DataEntryUrl,
     DataReference,
     DataInformation,
@@ -783,17 +890,25 @@ typedef testing::Types<
     Movie,
     TrackFragmentDecodeTime,
     MovieFragmentHeader,
-    TrackFragmentHeader,
-    TrackFragmentRun,
-    TrackFragment,
-    MovieFragment,
-    SegmentIndex> Boxes;
+    TrackFragmentHeader> Boxes;
 
 // GTEST support a maximum of 50 types in the template list, so we have to
 // break it into two groups.
 typedef testing::Types<
+    TrackFragmentRun,
+    TrackFragment,
+    MovieFragment,
+    SegmentIndex,
     SampleToGroup,
-    SampleGroupDescription> Boxes2;
+    SampleGroupDescription,
+    CueSourceIDBox,
+    CueTimeBox,
+    CueIDBox,
+    CueSettingsBox,
+    CuePayloadBox,
+    VTTEmptyCueBox,
+    VTTAdditionalTextBox,
+    VTTCueBox> Boxes2;
 
 TYPED_TEST_CASE_P(BoxDefinitionsTestGeneral);
 
diff --git a/packager/media/formats/mp4/fourccs.h b/packager/media/formats/mp4/fourccs.h
index 8f399281a0..8467003419 100644
--- a/packager/media/formats/mp4/fourccs.h
+++ b/packager/media/formats/mp4/fourccs.h
@@ -11,6 +11,7 @@ namespace edash_packager {
 namespace media {
 namespace mp4 {
 
+// TODO(rkuroiwa): Make these case sensitive. e.g. FOURCC_avc1.
 enum FourCC {
   FOURCC_NULL = 0,
   FOURCC_AVC1 = 0x61766331,
@@ -88,7 +89,9 @@ enum FourCC {
   FOURCC_STTS = 0x73747473,
   FOURCC_STYP = 0x73747970,
   FOURCC_STZ2 = 0x73747a32,
+  FOURCC_SUBT = 0x73756274,
   FOURCC_TENC = 0x74656e63,
+  FOURCC_TEXT = 0x74657874,
   FOURCC_TFDT = 0x74666474,
   FOURCC_TFHD = 0x74666864,
   FOURCC_TKHD = 0x746b6864,
@@ -97,8 +100,8 @@ enum FourCC {
   FOURCC_TREX = 0x74726578,
   FOURCC_TRUN = 0x7472756e,
   FOURCC_UDTA = 0x75647461,
-  FOURCC_URL  = 0x75726c20,
-  FOURCC_URN  = 0x75726e20,
+  FOURCC_URL = 0x75726c20,
+  FOURCC_URN = 0x75726e20,
   FOURCC_UUID = 0x75756964,
   FOURCC_VIDE = 0x76696465,
   FOURCC_VMHD = 0x766d6864,
@@ -107,6 +110,18 @@ enum FourCC {
   FOURCC_VP10 = 0x76703130,
   FOURCC_VPCC = 0x76706343,
   FOURCC_WIDE = 0x77696465,
+  FOURCC_ctim = 0x6374696d,
+  FOURCC_iden = 0x6964656e,
+  FOURCC_payl = 0x7061796c,
+  FOURCC_sthd = 0x73746864,
+  FOURCC_sttg = 0x73747467,
+  FOURCC_vlab = 0x766c6162,
+  FOURCC_vsid = 0x76736964,
+  FOURCC_vttC = 0x76747443,
+  FOURCC_vtta = 0x76747461,
+  FOURCC_vttc = 0x76747463,
+  FOURCC_vtte = 0x76747465,
+  FOURCC_wvtt = 0x77767474,
 };
 
 const inline std::string FourCCToString(FourCC fourcc) {