Progressive rock ("prog") is an ambitious, eclectic, and often grandiose style of rock music which arose in the late 1960s principally in England, reaching the peak of its popularity in the early 1970s, but continuing as a musical form to this day. Progressive rock was largely a European movement, and drew most of its influences from classical music and jazz fusion, in contrast to American rock, which was influenced by rhythm & blues and country, although there are notable exceptions in the New World such as Kansas and Rush — considered by many to be the finest examples of the form. Over the years various sub-genres of progressive rock have emerged, such as symphonic rock, art rock and progressive metal.

Progressive rock artists sought to move away from the limitations of radio formatted rock and pop, and "progress" rock to the point that it could achieve the sophistication of jazz or classical music. It is admired by its fans for its complexity, requiring a high level of musical virtuosity to perform. Critics have often derided the genre as pompous and self-indulgent. This is because, unlike such stylistically consistent genres as country or hip hop, progressive rock is difficult to define in a single conclusive way. Outspoken King Crimson leader Robert Fripp has voiced his disdain for the term. The major acts that defined the genre in the 1970s (Yes, Genesis, Emerson Lake and Palmer, Rush and King Crimson) do not sound alike. There is also debate on whether bands such as The Beatles, Phish, and Radiohead belong to the genre.

Some common, though not universal, elements of progressive rock include:
Long compositions, sometimes running over 20 minutes, with intricate melodies and harmonies that require repeated listening to grasp. These are often described as epics and are the genre's clearest nod to classical music. An early example is the 23-minute "Echoes" by Pink Floyd. Other famous examples include Jethro Tull's "Thick as a Brick" (43 minutes), Yes' "Close to the Edge" (18 minutes) and Genesis' "Supper's Ready" (23 minutes). More recent extreme examples are the 60-minute "Light of Day, Day of Darkness" by Green Carnation and "Garden of Dreams" by The Flower Kings.
Lyrics that convey intricate and sometimes impenetrable narratives, covering such themes as science fiction, fantasy, history, religion, war, love, and madness. Many early 1970s progressive rock bands (especially German ones) featured lyrics concerned with left-wing politics and social issues.
Concept albums, in which a theme or storyline is explored throughout an entire album in a manner similar to a film or a play. In the days of vinyl, these were usually two-record sets with strikingly designed gatefold sleeves. Famous examples include The Lamb Lies Down on Broadway by Genesis, Tales from Topographic Oceans by Yes, 2112 by Rush, Dark Side of the Moon and The Wall by Pink Floyd, and the more recent Metropolis Part II: Scenes from a Memory by Dream Theater and Snow by Spock's Beard. Aqualung, perhaps the best-known record by Jethro Tull, is often regarded as a concept album due to its recurring themes, but songwriter Ian Anderson has always claimed that the album is just "a bunch of songs".
Unusual vocal styles and use of multi-part vocal harmonies. See Magma, Robert Wyatt, and Gentle Giant.
Prominent use of electronic instrumentation — particularly keyboard instruments such as the organ, piano, Mellotron, and Moog synthesizer, in addition to the usual rock combination of electric guitar, bass and drums.
Use of unusual time signatures, scales, or tunings. Many pieces use multiple time signatures and/or tempi, sometimes concurrently. Solo passages for virtually every instrument, designed to showcase the virtuosity of the player. This is the sort of thing that contributed to the fame of such performers as keyboardist Rick Wakeman and drummer Neil Peart.