A style of rock music that explores unconventional territory, often incorporating non-standard chord progressions, tempo changes, unusual time signatures, avant garde passages and complex horn and orchestral arrangements. It generally relates to experimental bands and musicians influenced by 20th century avant garde, classical, jazz, folk and other music, and who stretch the boundaries of the rock form in both the compositional and improvisational arenas.

In the late 1970s, a group of European progressive bands (including Henry Cow and Slapp Happy) joined forces under the banner of Rock in Opposition (RIO) to unite against the music industry that refused to let their music be heard. While RIO was originally a collective of bands with the same political agenda, the term is now often used to identify the 'collective' style of music of the original RIO bands, which later became known as avant-progressive rock although this term now encompasses a far greater body of music.

Other bands often included in this category include Captain Beefheart and the Magic Band and Frank Zappa (with and without the Mothers of Invention). More modern bands include Fantômas (avant garde metal), Mr. Bungle and Faith No More (avant garde experimental rock) - all featuring Mike Patton.