Avant-prog is a music genre characterized by highly complex, dense structures often featuring dissonance and uncommon time signatures, sometimes extending towards atonality, free time, Free Improvisation, and other experimental techniques. The music tends to be more abstract, energetic, and playful, with more extreme cases generating an ominous ambiance and suspense. It is usually compared with Progressive Rock due to both being Rock subgenres that emerged around the same time and focused on intricate arrangements and technical interplay.

Avant-prog's origins date back to the late 1960s, specifically Jazz-Rock and Frank Zappa's work with The Mothers of Invention, who implemented orchestral instrumentation, playful, intricate arrangements, and avant-garde tendencies. Canterbury Scene artists such as Soft Machine were also influential due to their increasingly experimental blend of jazz-rock, Jazz Fusion, and Psychedelic Rock. Additionally, various Modern Classical and Avant-Garde Jazz musicians like Igor Stravinsky and Ornette Coleman influenced early avant-prog acts with their highly experimental approach.

Initial avant-prog bands originated in the early 1970s in Western Europe, with various groups pioneering the sound and developing respective scenes. Some of the more prominent groups, including Henry Cow, Univers Zéro, and Samla Mammas Manna, would later associate themselves under the international Rock in Opposition collective, which was formed to promote music that, due to its experimental nature, didn't generate enough revenue for mainstream record labels. Meanwhile, Magma developed Zeuhl, a more orchestral, Jazz-oriented style that featured the usage of artificial language called Kobaïan, operatic vocals, and march influences. It later spread outside the French scene and became an essential influence on Brutal Prog, which emerged in Japan and eventually arrived in the US.