Underground music comprises musical genres beyond mainstream culture. Any song that is not being legally commercialized is considered underground. Such music may tend to express common ideals, such as high regard for sincerity and intimacy, freedom of creative expression as opposed to the highly formulaic composition of commercial music, and appreciation of artistic individuality as opposed to conformity to current mainstream trends. Apart from perhaps the underground rock scenes in the pre-Mikhail Gorbachev Soviet Union, very few types of underground music are completely hidden, although performances and recordings may be difficult for outsiders to find.

Some underground musical genres never left their non-mainstream roots, such as jagged, aggressive UK 82-style hardcore punk bands like Discharge. Some underground styles eventually became mainstream, commercialized pop styles, as did for example the underground hip hop style of the early 1980s. In the 2000s, the increasing availability of the Internet and digital music technologies has made underground music easier to distribute using streaming audio and podcasts. Some experts in cultural studies now argue that "there is no underground" because the Internet has made what was underground music accessible to everyone at the click of a mouse. One expert, Martin Raymond, of London-based company The Future Laboratory commented in an article in The Independent, saying trends in music, art and politics are: