Black Flag was an American punk rock band formed in 1976 in Hermosa Beach, California. The band was established largely as the brainchild of Greg Ginn: the guitarist, primary songwriter and sole continuous member through multiple personnel changes. They are widely considered to be one of the first hardcore punk bands.

Black Flag forged a unique sound early on that mixed the raw simplicity of the Ramones with atonal guitar solos and frequent tempo shifts. The band was also known for the intense and evocative lyrics found in their songs, most of which were penned by Ginn. Like other punk bands of this era, Black Flag gave voice to an anti-authoritarian, non-conformist message, featuring songs punctuated with descriptions of isolation, neurosis, poverty, and paranoia, themes that would be explored further when Henry Rollins joined the group as lead singer in 1981. Most of the band's material was released on Ginn's independent label, SST Records.

Black Flag was—and remains—well respected among their underground culture, with their influence primarily in their tireless promotion of a self-controlled DIY ethic and aesthetic. They are often regarded as pioneers in the movement of underground do-it-yourself record labels that flourished among the 1980s punk rock bands. Through seemingly constant touring throughout the United States and Canada, and occasionally Europe, Black Flag established an extremely dedicated fan base. Many other musicians would follow Black Flag's lead and book their own tours, utilizing a word-of-mouth network.

Over the course of the 1980s, Black Flag's sound, as well as their notoriety, evolved in ways that alienated much of their early punk audience. As well as being central to the creation of hardcore, they were part of the first wave of American West Coast punk rock and are considered a key influence on the punk subculture. Along with being among the earliest punk rock groups to incorporate elements and the influence of heavy metal melodies and rhythm (particularly in their later records), there were often overt freestyles, free jazz, breakbeat and contemporary classical elements in their sound, especially in Ginn's guitar playing, and the band interspersed records and performances with instrumentals throughout their career. They also played longer, slower, and more complex songs at a time when many bands in their milieu stuck to a raw, fast, three-chord format. As a result, their extensive discography is more stylistically varied than many of their punk rock contemporaries