Jethro Tull are a British rock group formed in 1967-1968. Their music is marked by the distinctive vocal style and lead flute work of front man Ian Anderson. Initially playing blues rock with an experimental flavour, they incorporated elements of classical, folk and 'ethnic' musics, jazz and art rock into their music.

Ian Anderson's first band, started in 1963 in Blackpool, was known as The Blades. By 1966, they had developed into a seven-piece white soul band called the John Evan Band (later the John Evan Smash), named for pianist/drummer John Evans, who dropped the final "s" from his name to make it sound less ordinary. At this point, Barriemore Barlow was the band's drummer, as he would later be for Jethro Tull itself.

The band moved to the London area in search of more bookings, basing themselves in nearby Luton. They also traveled to Liverpool. However, money remained short and within days of the move most of the band quit and headed back north, leaving Anderson and bassist Glenn Cornick to join forces with blues guitarist Mick Abrahams and his friend, drummer Clive Bunker, both from the Luton-based band McGregor's Engine.

At first, the new band had trouble getting repeat bookings and they took to changing their name frequently to continue playing the London club circuit. Band names were often supplied by their booking agents' staff, one of whom, a history enthusiast, eventually christened them Jethro Tull after the 18th-century agriculturist.

This name stuck simply by virtue of the fact that they were using it the first time a club manager (namely, John Gee of the Marquee Club, London) liked their show enough to invite them to return. They were signed to the blossoming Ellis-Wright agency, and became the third band managed by the soon-to-be Chrysalis empire.

Their first single, produced by Derek Lawrence, was called "Sunshine Day". It was penned by Abrahams. In this single the group's name was misspelled "Jethro Toe", making it a collector's item. "Sunshine Day" was unsuccessful.

They next released the bluesy album This Was in 1968. In addition to music written by Anderson and Abrahams the album included the traditional "Cat's Squirrel", which highlighted Abraham's blues-rock style. The Rahsaan Roland Kirk-penned jazz piece "Serenade to a Cuckoo" gave Anderson a showcase for his growing talents on the flute, an instrument which he started learning to play only half a year before the release of the album. The overall sound of the group at this time was described in the Record Mirror by Anderson in 1968 as "a sort of progressive blues with a bit of jazz".

Following this album, Abrahams left after a falling out with Anderson and formed his own band, Blodwyn Pig. There were a number of reasons for his departure: he was a blues purist, while Anderson wanted to branch out into other forms of music; Abrahams and Cornick did not get along; and Abrahams was unwilling to travel internationally or play more than three nights a week, while the others wanted to be successful by playing as often as possible and building an international fan base.

Guitarist Tony Iommi, from the group Earth (who would soon change their name to Black Sabbath), took on guitar duties for a short time after the departure of Abrahams, appearing in The Rolling Stones Rock and Roll Circus (in which the group mimed "A Song For Jeffrey") in December 1968, but it turned out to be a one time only arrangement and Tony returned to Earth after the performance.

After auditions for a replacement guitarist in December 1968, Anderson chose Martin Barre, a former member of Motivation, Penny Peeps, and Gethsemane, who was playing with Noel Redding's Fat Mattress at the time. Barre impressed Anderson with his persistence more than anything else: he was so nervous at his first audition that he could hardly play at all, and then showed up for a second audition without an amplifier or a cord to connect his guitar to another amp. Nevertheless, Barre would become Abrahams' permanent replacement on guitar and the second longest-standing member of the band after Anderson.

This new line-up released Stand Up in 1969, the group's only UK number-one album. Written entirely by Anderson, with the exception of the jazzy rearrangement of J. S. Bach's Bourée (fifth movement from Suite for Lute in E minor BWV 996 (BC L166)), it branched out further from the blues, clearly evidencing a new direction for the group, which would come to be categorised as progressive rock alongside such diverse groups as King Crimson, Genesis, The Nice and Yes.

It was during sessions for this album that the band recorded their most well-known song, "Living in the Past", which originally ended up being issued as a stand-alone single. Anderson and Chrysalis Records manager Terry Ellis reportedly wrote it in 5/4 time with the intent of preventing its ascent to the pop charts.

It turned out not to be the case, as the song reached number three in the UK chart, and though most other progressive groups actively resisted issuing singles at the time, Jethro Tull had further success with their other singles, "Sweet Dream" (1969) and "The Witch's Promise" (1970), and a five-track EP, Life Is a Long Song (1971), all of which made the top twenty. In 1970, they added keyboardist John Evan (initially as a guest musician) and released the album Benefit.

Bassist Cornick left following Benefit, and formed the band "Wild Turkey".He was replaced by Jeffrey Hammond, a childhood friend of Anderson whose name appeared in the songs "A Song for Jeffrey", "Jeffrey Goes to Leicester Square", "For Michael Collins, Jeffrey, and Me", and who also is the writer and narrator of "The Story of the Hare Who Lost His Spectacles", later featured in the album A Passion Play. Hammond was often credited on Jethro Tull albums as "Jeffrey Hammond-Hammond", but the extra "Hammond" was an inside joke regarding the fact that Hammond's mother's maiden name was also "Hammond", no relation to his father.

This line-up released Jethro Tull's best-known work, Aqualung in 1971. On this album, Anderson's writing voiced strong opinions about religion and society. Though consisting of distinct tracks, there is a common narrative thread leading some to label it as a concept album. The title character of Aqualung is a disreputable tramp, wandering the streets and "eyeing little girls with bad intent"; the focus of the song "Cross-Eyed Mary" is a young prostitute who operates from near a school.

"My God", written before Benefit and already a staple of the band's live act before Aqualung's release, is a full-frontal assault on ecclesiastic excesses: "People what have you done/locked Him in His golden cage/Made Him bend to your religion/Him resurrected from the grave..." In contrast, the gentle acoustic "Wond'ring Aloud" is a love song. The title track and "Locomotive Breath" remain staples of U.S. classic rock stations and, to this day, are rarely left out of Jethro Tull's live act.

Because of the heavy touring schedule and his wish to spend more time with his family, drummer Bunker quit the group after the Aqualung album, and was replaced by Barriemore Barlow in early 1971. Barlow first recorded with the band for the EP Life Is a Long Song and made his first appearance on a Jethro Tull album with 1972's Thick as a Brick.

This was conceived as a concept album consisting of a single track running 43:28 (an innovation previously unheard of in rock music), split over the two sides of the LP, with a number of movements melded together and some repeating themes. The first movement with its distinctive acoustic guitar riff received some airplay on rock stations at the time (and occasionally turns up in modern classic-rock programming as a "deep" or "rare" cut).

Thick as a Brick was the first true prog rock offering by the band, as well as the first Jethro Tull album to reach number one on the (U.S.) Billboard Pop Albums chart (the following year's A Passion Play being the only other). This album's quintet – Anderson, Barre, Evan, Hammond, and Barlow – endured until the end of 1975.

1972 also saw the release of Living in the Past, a double-album compilation of remixed singles, B-sides and outtakes (including the entirety of the Life Is a Long Song EP, which closes the album), with a single side recorded live in 1970 at New York's Carnegie Hall. Fans regard the album as arguably the band's best compilation. The title song, recorded and released three years earlier, would gain even greater U.S. success because of this album.

In 1973, the band attempted to record a double album in tax exile at France's Château d'Hérouville studios (something the Rolling Stones and Elton John among others were doing at the time), but supposedly they were unhappy with the quality of the recording studio and abandoned the effort, subsequently mocking the studio as the "Chateau d'Isaster." (An 11-minute excerpt from these recordings was released on the 1988 20 Years of Jethro Tull boxed set, and the complete sessions were finally released on the 1993 compilation Nightcap, with the contemporarily overdubbed flute lines where the vocal parts were missing.)

Instead they returned to England and Anderson rewrote, quickly recorded, and released A Passion Play, another single-track concept album, and their second pure prog release, with very allegorical lyrics focusing on the afterlife. A Passion Play continued the diverse instrumentation introduced in Thick As a Brick, and added saxophones to the mix. A Passion Play sold well but received generally poor reviews, including a particularly damning review of its live performance by Chris Welch of Melody Maker.

Around this time, the band's popularity with critics began to wane, but their popularity with the public remained strong. 1974's War Child, an album originally intended to be a companion piece for a film, reached number two on the Billboard charts and received some critical acclaim, and produced the radio mainstays "Bungle in the Jungle" and "Skating Away (On the Thin Ice of the New Day)". It also included a song, "Only Solitaire", allegedly aimed at L.A. Times rock music critic Robert Hilburn, who was one of Anderson's harsher critics. The War Child tour also featured a female string quartet playing along with the group on the new material.

In 1975, the band released Minstrel in the Gallery, an album which resembled Aqualung in that it contrasted softer, acoustic guitar-based pieces with lengthier, more bombastic works headlined by Barre's electric guitar. Written and recorded during Anderson's divorce from his first wife Jennie Franks, the album is characterised by introspective, cynical, and sometimes bitter lyrics.

Critics gave it mixed reviews, but the album came to be acknowledged as one of the band's best by longtime Jethro Tull fans, even as it generally fell under the radar to listeners familiar only with Aqualung. For the 1975 tour, David Palmer, who had long been the band's orchestra arranger, formally joined the band on keyboards and synthesizers. After the tour, bassist Hammond quit the band to pursue painting. John Glascock, who earlier was playing with flamenco-rock band Carmen, a support band on the previous Jethro Tull tour, was tabbed as the band's new bassist.

1976's Too Old to Rock 'n' Roll: Too Young to Die! was another concept album, this time about the life of an aging rocker. Anderson, stung by critical reviews (particularly of A Passion Play), responded with more sharply-barbed lyrics. The press seemed oblivious to the ploy, and instead asked if the title track was autobiographical — a charge Anderson hotly denied. Curiously, the sleeve for the album featured a comic strip with a lead character (Ray Lomas) that looked very similar to Anderson.