libavcodec is a free and open-source[4] library of codecs for encoding and decoding video and audio data.

The popular MPV, xine and VLC media players use it as their main, built-in decoding engine that enables playback of many audio and video formats on all supported platforms.

Having such codecs available within the standard libavcodec framework gives a number of benefits over using the original codecs, most notably increased portability, and in some cases also better performance, since libavcodec contains a standard library of highly optimized implementations of common building blocks, such as DCT and color space conversion.

However, while libavcodec does strive to achieve decoding that is bit-exact to their official format implementations, occasional bugs and missing features in such re-implementations can sometimes introduce playback compatibility problems for certain files.

For example, some European Union nations have not implemented software patents and have laws expressly allowing reverse engineering for purposes of interoperability.