Master of Reality is the third studio album by the English heavy metal band Black Sabbath, released on 6 August 1971 by Vertigo Records. It is regarded by some critics as the foundation of stoner rock and sludge metal. Produced by Rodger Bain, who also produced the band's prior two albums, Master of Reality was recorded at Island Studios in London from February to April 1971. This would end up being their last album produced by Bain, as production duties would be taken over by guitarist Tony Iommi for their subsequent albums.

Iommi and bassist Geezer Butler downtuned their instruments during the production, achieving what Iommi called a "bigger, heavier sound". The band generally approached the album with a more experimental lens, partly due to having more time to record and write than on their previous albums.

Master of Reality peaked at number five on the UK Albums Chart and number eight on the US Billboard 200. Though negatively received by critics on release, the album is now considered one of the greatest heavy metal albums of all time. It was certified double platinum by the Recording Industry Association of America (RIAA) after having sold over two million copies in the US.

By 1971, Sabbath had become a highly successful band in the UK and US, and were considered, along with Led Zeppelin and Deep Purple, part of the "unholy trinity of British hard-rock and heavy metal". Despite negative critical perceptions of their music, both their eponymous debut and their sophomore album Paranoid were commercially successful in both the US and the UK. In February 1971, after a one-off performance at the Myponga Pop Festival in Australia, Black Sabbath returned to the studio to begin work on their third album.

Master of Reality was recorded at Island Studios in London from February to April 1971. The album was produced by Rodger Bain, who had also produced Black Sabbath's previous two albums, with Tom Allom handling engineering. This was to be Bain's final collaboration with Black Sabbath as guitarist Tony Iommi took over production for the band's next several albums. Drummer Bill Ward explained: "Previously, we didn't have a clue what to do in the studio, and relied heavily on Rodger. But this time we were a lot more together, understood what was involved and were more opinionated on how things should be done."

On the tracks "Children of the Grave", "Lord of This World", and "Into the Void", Iommi downtuned his guitar 11⁄2 steps in an effort to reduce string tension, thus making the guitar less painful for him to play. This pain was the result of a factory accident years earlier in which he had the tips of two of his fingers severed. The downtuning also helped the guitarist produce what he called a "bigger, heavier sound". Geezer Butler also downtuned his bass guitar to match Iommi. "It helped with the sound, too", Butler explained to Guitar for the Practicing Musician in 1994. "Then it got to the point where we tuned even lower to make it easier vocal-wise. But Ozzy (Osbourne) would then sing higher so it sort of defeated the object."

In the 2013 biography of the band Black Sabbath: Symptom of the Universe, Mick Wall writes that "the Sabbath sound took a plunge into even greater darkness. Bereft even of reverb, leaving their sound as dry as old bones dug up from some desert burial plot, the finished music's brutish force would so alarm the critics they would punish Sabbath in print for being blatantly thuggish, purposefully mindless, creepy, and obnoxious. Twenty years later groups like Smashing Pumpkins, Soundgarden, and, particularly, Nirvana, would excavate the same heaving lung sound ... And be rewarded with critical garlands." In his autobiography I Am Ozzy, vocalist Osbourne states that he cannot remember much about recording Master of Reality "apart from the fact that Tony detuned his guitar to make it easier to play, Geezer wrote 'Sweet Leaf' about all the dope we'd been smoking, and 'Children of the Grave' was the most kick-ass song we'd ever recorded."

In the liner notes to the 1998 live album Reunion, drummer Ward commented that Master of Reality was "an exploratory album". Ward elaborated in a 2016 interview with Metal Hammer magazine: "On the first album, we had two days to do everything, and not much more time for Paranoid. But now we could take our time, and try out different things. We all embraced the opportunity: Tony threw in classical guitar parts, Geezer's bass was virtually doubled in power, I went for bigger bass drums, also experimenting with overdubs. And Ozzy was so much better. But this was the first time when we didn't have gigs booked in, and could just focus on making the album a landmark." In 2013, Mojo magazine called Master of Reality "The sound of a band becoming increasingly comfortable in their studio surroundings."