Jethro Tull are a British progressive rock band formed in Blackpool, Lancashire, in 1967. Initially playing blues rock and jazz fusion, the band soon incorporated elements of English folk music, hard rock and classical music, forging a signature progressive rock sound. The group's lead vocalist, bandleader, founder, principal composer and only constant member is Ian Anderson, who also plays flute and acoustic guitar. The group has featured a succession of musicians throughout the decades, including significant contributors such as guitarists Mick Abrahams and Martin Barre (with Barre being the longest-serving member besides Anderson); bassists Glenn Cornick, Jeffrey Hammond, John Glascock, Dave Pegg, Jonathan Noyce and David Goodier; drummers Clive Bunker, Barrie "Barriemore" Barlow and Doane Perry; and keyboardists John Evan, Dee Palmer, Peter-John Vettese, Andrew Giddings and John O'Hara.

The band achieved moderate recognition in the London club scene and released their debut album, This Was, in 1968. After a line-up change which saw original guitarist Mick Abrahams replaced by Martin Barre, the band released a folk-tinged second album, Stand Up, in 1969. Stand Up, which reached No. 1 in the UK, gave the band their first commercial success, and regular tours of the UK and the US followed. Their musical style shifted in the direction of progressive rock with albums such as Aqualung (1971), Thick as a Brick (1972), and A Passion Play (1973), and shifted again to contemporary folk rock with Songs from the Wood (1977), Heavy Horses (1978), and Stormwatch (1979). In the early 1980s, the band underwent a major line-up change and moved into electronic rock with the albums A (1980), The Broadsword and the Beast (1982), and Under Wraps (1984). The band won their sole Grammy Award for the 1987 album Crest of a Knave, which saw them returning to a hard rock style. Jethro Tull have sold an estimated 60 million albums worldwide, with 11 gold and 5 platinum albums. They have been described by Rolling Stone as "one of the most commercially successful and eccentric progressive rock bands".

The band ceased studio recording activity in the 2000s, but continued to tour until splitting in 2011. Following the band's split, Anderson and Barre continued to record and tour as solo artists, with Anderson's band billed variously as both "Jethro Tull" and "Ian Anderson" solo. Anderson said in 2014 that Jethro Tull had come "more or less to an end". In 2017, however, Anderson revived the Jethro Tull name and released new studio albums in the 2020s. The current group includes musicians who were part of Jethro Tull during the last years of its initial run, as well as newer musicians associated with Anderson's solo band, without Barre's involvement.