Aqualung was the fourth studio album by prog rock band Jethro Tull. Released in 1971, Aqualung, despite the band's disapproval, is regarded as a concept album featuring a central theme of "the distinction between religion and God". The album's "dour musings on faith and religion" have marked it as "one of the most cerebral albums ever to reach millions of rock listeners". Aqualung 's success marked a turning point in the band's career, who went on to become a major radio and touring act. There's no arguing with it's commercial success, having sold more than seven million copies and continuing to outsell anything in the back catalog.