Progressive rock, often shortened to prog rock or prog, is a form of rock music characterized by lengthy and complicated forms (often based on classical or jazz, and frequently consisting of multiple contrasting sections), the use of instruments not typically found in rock, and complicated harmonies and rhythms. Other defining features include the "concept album" (such as Dark Side of the Moon or The Wall), fantasy-based lyrics, and artistic album covers.
Progressive rock arose during the late 1960s and carried on through the 1970s before a decline corresponding to the rise of punk. King Crimson, Yes, Emerson, Lake, & Palmer, and Genesis are considered the four "classic" exponents of this genre, although bands as diverse as Pink Floyd, Jethro Tull, and Rush have been given this label.
The genre has been criticized for some of its excesses and pretensions, which led to its downfall in the late 1970s (although bands like Pink Floyd and Rush continued to maintain a large following). This could be seen as a general turn in popular music towards more commercial and easily-accessible forms (such as disco and smooth jazz).
In the 1980s, many "neo-prog" artists such as Pallas and Marillion began a revival of the genre, combining the virtuosity and complexity of the '70s bands with a more mainstream sound reflecting the New Wave movement. Stalwarts of the genre (including Yes and Genesis) also modified their sound to head in a more mainstream direction. One example of this is the formation of the supergroup Asia, featuring former members of Yes, King Crimson, and ELP. Asia's pop-oriented debut established a sales record for 1982, but surprised disappointed many fans of progressive rock who were expecting something less mainstream.
In the 1990s a new wave of "progressive metal" bands arose who combined the sound of traditional prog bands with metal bands such as Black Sabbath and Metallica. The most prominent of these is Dream Theater, while other such bands include Symphony X, Queensryche, and Tool.
In recent times the genre has enjoyed a renaissance with the formation and popularity of bands such as Porcupine Tree and The Pineapple Thief, as well as other, more diverse sub-genres, such as Opeth's technical death styled progressive metal and Frost, playing eclectic progressive rock.