MPEG-4 Part 14 or *.mp4, is a file format (a so called container) specified as a part of the ISO/IEC MPEG-4 international standard. It is used to store media types defined by the ISO/IEC Moving Picture Experts Group, and can be used to store other media types as well. It is, or will be, typically used to store data in files, though it will be used in data streams and possibly in other ways.

*.mp4 allows streaming over the internet. It also allows multiplexing of multiple video and audio streams in one file, variable frame- and bit-rates, subtitles and still images. Apple's QuickTime file format was the start point design of the *.mp4 file format. Although still very reminiscent of each other, *.mp4 has changed and been improved in many different ways apart from *.mov that they are really not the same file format anymore.

*.mp4 is most commonly used as the alternative to *.mp3 on an Apple iPod and in iTunes. The quality of the AAC codec that is stored in *.mp4 is higher than MPEG-1 Audio Layer 3 , although not as widely used in computer and hardware players as the *.mp3.

Approximately any kind of data can be embedded in *.mp4 files through private streams, but those that are recommended (for compatibility reasons) are:

• Video: MPEG-4, MPEG-2 and MPEG-1
• Audio: MPEG-4 AAC, MP3, MP2, MPEG-1 Part 3, MPEG-2 Part 3, CELP (speech), TwinVQ (very low bitrates), SAOL (midi)
• Pictures: JPEG, PNG
• Subtitles: MPEG-4 Timed Text, and/or xmt/bt text format (means that subtitles have to be translated into xmt/bt)

Some file extensions used on files that contain data in the *.mp4 format:

• .mp4: official extension, for audio, video and advanced content (see above) files
• .m4a: for audio-only files; can safely be renamed to .mp4, though opinions differ on the wisdom of this.
• .m4p: FairPlay protected files
• .mp4v , .m4v : video-only (sometimes also used for raw mpeg-4 video streams not in the *.mp4 container format)
• .3gp, .3g2 : used by 3G mobile phones, may also store content not specified directly in the .mp4 specification (H.263, AMR, TX3G)