Flash Video (FLV) is a proprietary file format used to deliver video over the Internet using Adobe Flash Player (formerly known as Macromedia Flash Player) version 6, 7, 8, or 9. Flash Video content may also be embedded within SWF files. Notable users of the Flash Video format include YouTube, Google Video, Reuters.com, Yahoo! Video and MySpace. The BBC have recently begun using .flv based media on their news portal.
Flash Video is viewable on most operating systems, via the widely available Adobe Flash Player and web browser plugin, or one of several third-party programs such as MPlayer, VLC media player, or any player which uses DirectShow filters (such as Media Player Classic, Windows Media Player, and Windows Media Center) when the ffdshow filter is installed.

Flash Player
Commonly, Flash Video files contain video bit streams which are a variant of the H.263 video standard, under the name of Sorenson Spark. Flash Player 8 and newer revisions support the playback of On2 TrueMotion VP6 video bit streams. On2 VP6 can provide a higher visual quality than Sorenson Spark, especially when using lower bit rates. On the other hand it is computationally more complex and therefore will not run as well on certain older system configurations.
An optional alpha channel which represents per pixel transparency is supported by including a second simultaneous video stream which encodes the alpha channel only, dropping any chromatic information. The implementation makes the assumption that the YUV data of the main On2 VP6 video stream is always converted to RGB by the client before compositing occurs as the resulting RGB values are alpha premultiplied and clamped accordingly. This option is only available for On2 VP6 encoded video streams.
The Flash Video file format supports two versions of a so called 'screenshare' codec which is an encoding format designed for screencasts. Both these formats are bitmap tile based, can be lossy by reducing color depths and are compressed using zlib. The second version is only playable in Flash Player 8 and newer.
Support for encoding Flash Video files is provided by an encoding tool included with Adobe's Macromedia Flash Professional 8 product, On2's Flix encoding tools, Sorenson Squeeze, FFmpeg and other third party tools.

Video format
Audio in Flash Video files is usually encoded as MP3. However, Flash Video files recorded from the user's microphone use the proprietary Nellymoser codec. There is currently no open source product available that can decode the nellymoser codec. FLV files also support uncompressed audio or ADPCM format audio.