A bootleg recording is an audio (or video) recording of a live performance or even an interview that was not officially released by the artist or under other legal authority.

Bootlegs can consist of recordings of live performances or material created in private or professional recording sessions. Changing technologies have had a great impact on the recording, distribution, and varying profitability of the underground industry. In the 70s bootlegs were mainly on vinyls, and years later the audio cassette greatly increased the distribution of bootleg recordings.
In the 1990s, there was a widespread conversion of many of the older bootlegs onto the compact disc format. Unofficial recordings became more readily available than ever before, resulting in thousands of bootlegs being circulated on CD amongst avid collectors and fans, in many cases of shows which had been originally recorded over thirty years previously.
However, the late 1990s and early 2000s saw an increase in the free trading of digital bootlegs, sharply decreasing the demand for and profitability of physical bootlegs. The rise of standard audio file formats such as MP3 and FLAC, combined with the ability to share files between computers via e-mail, FTP, instant messaging, and specialized peer-to-peer file sharing networks such as Napster and BitTorrent, made it simpler than ever for bootleg collectors to exchange rarities. Older analog recordings were converted to digital format for the first time, tracks from bootleg CDs were ripped to computer hard disks, and new material was created with digital recording of various types, and all of these types could now be easily shared. The quality and portability of recording devices and microphones also increased exponentially, resulting in recordings which were often on a par with official releases. One notable change caused by this shift in technology was the unit of exchange: instead of album-length collections or live recordings of entire shows, fans often now had the option of searching for and downloading bootlegs of individual songs.

Legend: Album title – Record Company – Catalogue number ~ Notes