Metallica /məˈtælɨkə/ is an American heavy metal band from Los Angeles, California. The band's fast tempos, instrumentals, and aggressive musicianship placed them as one of the founding "big four" of thrash metal alongside Slayer, Megadeth, and Anthrax. Metallica formed in 1981 when James Hetfield responded to an advertisement that drummer Lars Ulrich had posted in a local newspaper. Since 2003, the line-up features rhythm guitarist and vocalist Hetfield, drummer Ulrich, lead guitarist Kirk Hammett and bassist Robert Trujillo. Previous members of the band are lead guitarist Dave Mustaine (who went on to found Megadeth), and bassists Ron McGovney, Cliff Burton and Jason Newsted. The band also had a long collaboration with producer Bob Rock, who produced all of its albums from 1990 to 2003 and served as a temporary bassist between the departure of Newsted and the hiring of Trujillo.

The band earned a growing fan-base in the underground music community and critical acclaim with its first four albums, with their third, Master of Puppets (1986), described as one of the most influential and "heavy" thrash metal albums. Metallica achieved substantial commercial success with their eponymous fifth album (also known as The Black Album), which debuted at number one on the Billboard 200. With this release the band expanded its musical direction resulting in an album that appealed to a more mainstream audience.

In 2000, Metallica was among a number of artists who filed a lawsuit against Napster for sharing the band's copyright-protected material for free without any band member's consent. A settlement was reached, and Napster became a pay-to-use service. Despite reaching number one on the Billboard 200, the release of St. Anger (2003) alienated many fans with the exclusion of guitar solos and the "steel-sounding" snare drum. A film titled Some Kind of Monster documented the recording process of St. Anger and the tensions within the band during that time. In 2009, Metallica was inducted into the Rock and Roll Hall of Fame.

Metallica has released nine studio albums, four live albums, five extended plays, 25 music videos, and 37 singles. The band has won nine Grammy Awards, and has had five consecutive albums debut at number one on the Billboard 200, making Metallica the first band to do so. The band's 1991 album, Metallica, has sold over 15.8 million copies in the United States, making it the best-selling album of the SoundScan era. Metallica ranks as one of the most commercially successful metal bands in history, having sold over 180 million albums worldwide as of 2013. Metallica has also been listed and ranked as one of the greatest bands of all time by many magazines, including Rolling Stone magazine which ranked them 61th on its list of The 100 Greatest Artists of All Time. As of December 2012, Metallica is the fourth best-selling music artist since the SoundScan era began tracking sales on May 25, 1991, selling a total of 53,642,000 albums in the United States alone.

In 2012, Metallica formed the independent record label Blackened Recordings, and have acquired the rights to all of their studio albums.