Black Sabbath were an English rock band formed in Birmingham in 1968 by guitarist Tony Iommi, drummer Bill Ward, bassist Geezer Butler, and vocalist Ozzy Osbourne. They are often cited as pioneers of heavy metal music. The band helped define the genre with their first three albums Black Sabbath, Paranoid (both 1970), and Master of Reality (1971). Following Osbourne's departure in 1979, the band underwent multiple line-up changes, with Iommi being the only constant member throughout its history.

After previous iterations of the group – the Polka Tulk Blues Band, and Earth – the band settled on the name Black Sabbath in 1969. They distinguished themselves through occult themes with horror-inspired lyrics and down-tuned guitars. Signing to Philips Records in November 1969, they released their first single, a cover of the Crow song "Evil Woman", in January 1970, and their debut album, Black Sabbath, was released the following month. Though it received a negative critical response, the album was a commercial success, leading to a follow-up record, Paranoid, later that year. The band's popularity grew, and by 1973's Sabbath Bloody Sabbath, critics were starting to respond favourably. This album, along with its predecessor Vol. 4 (1972) and its successors Sabotage (1975), Technical Ecstasy (1976) and Never Say Die! (1978), saw the band explore more experimental and progressive styles.

Osbourne's excessive substance abuse led to his firing in 1979. He was replaced by former Rainbow vocalist Ronnie James Dio. Sabbath recorded three albums with Dio: Heaven and Hell (1980), Mob Rules (1981) and the live album Live Evil (1982), the last two featuring drummer Vinny Appice replacing Ward. Following Dio and Appice's departures, Iommi and Butler recorded Born Again (1983) with then-former Deep Purple vocalist Ian Gillan and Ward returning on drums, while the latter was replaced by then-Electric Light Orchestra drummer Bev Bevan on the subsequent tour. Black Sabbath split in 1984, with Iommi assembling a new version of the band the following year. For the next twelve years, the band endured many personnel changes that included vocalists Glenn Hughes (who appeared on 1986's Seventh Star), Ray Gillen and Tony Martin, as well as several drummers and bassists; of the vocalists during these years, Martin's tenure was the longest, joining the band in 1987 and recording three studio albums before his initial departure in 1991: The Eternal Idol (1987), Headless Cross (1989), and Tyr (1990). That same year, Iommi reunited with Butler, Dio and Appice to record Dehumanizer (1992), though Dio and Appice had both departed again by the end of 1992. Martin returned as vocalist for two more studio albums, Cross Purposes (1994) and Forbidden (1995), and one live album before the band went on a one-year hiatus during 1996. The original line-up of Iommi, Osbourne, Butler and Ward reunited in 1997, releasing a live album, Reunion, in the following year, and continuing to tour sporadically until 2005, most of which saw Black Sabbath headline Osbourne's annual festival tour Ozzfest. The band went on another hiatus in 2006 when the Mob Rules line-up reunited as Heaven & Hell, touring during the late 2000s and releasing its sole studio album, The Devil You Know, in 2009 before disbanding after Dio's death in the following year. The original line-up of Black Sabbath reunited once again in 2011, though Ward departed prior to the recording of their final studio album and 19th overall, 13 (2013). During their farewell tour, Black Sabbath played its final concert in their home city of Birmingham on 4 February 2017. Occasional partial reunions have occurred since, most recently when Osbourne and Iommi performed together at the closing ceremony of the 2022 Commonwealth Games in Birmingham.

Black Sabbath have sold over 70 million records worldwide as of 2013, making them one of the most commercially successful heavy metal bands. Black Sabbath, together with Deep Purple and Led Zeppelin, have been referred to as the "unholy trinity of British hard rock and heavy metal in the early to mid-seventies". They were ranked by MTV as the "Greatest Metal Band of All Time" and placed second on VH1's "100 Greatest Artists of Hard Rock" list. Rolling Stone magazine ranked them number 85 on its "100 Greatest Artists of All Time" list. Black Sabbath were inducted into the UK Music Hall of Fame in 2005 and the Rock and Roll Hall of Fame in 2006. They have also won two Grammy Awards for Best Metal Performance, and in 2019, the band received a Grammy Lifetime Achievement Award.