Black Flag was a hardcore punk band formed in 1977 in southern California, largely as the brainchild of Greg Ginn: the guitarist, primary songwriter and sole continuous member through multiple personnel changes. They are widely considered the first hardcore punk band.

Black Flag forged a unique sound early on that mixed the raw simplicity of the Ramones with atonal and microtonal guitar solos and frequent tempo shifts. Over this could be heard lyrics—mostly written by Ginn—about isolation, neurosis, poverty, and paranoia, themes which did not disappear when Henry Rollins took on the role of lead singer in 1981. Most of the band's material was released on Ginn's independent label, SST Records.

Black Flag were (and remain) well respected among their underground culture, with their influence primarily in their tireless promotion of a self-controlled DIY ethic and aesthetic. They're 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 of heavy metal (particularly in their later records), there were often overt freestyles, jazz (mainly 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, Black Flag's extensive discography is more varied than many of their punk-rock contemporaries.