When FM broadcasting licenses were first issued by the FCC, broadcasters were slow to take advantage of the new airwaves available to them because their advertising revenues were generated primarily from existing AM broadcasting stations and because there were few FM radio receivers owned by the general public. This void created an opportunity for the disenchanted youth counter-culture of the era to express itself by playing music that was largely ignored by mainstream outlets. In this sense, progressive rock radio was more of a social response than a product marketed to fill a need.
This change coincided with the greater emphasis on albums as opposed to singles in the rock market. Hugely popular albums such as The Beatles' Sgt. Pepper's Lonely Hearts Club Band did not contain any singles, so there was clearly a need for a radio format that would explore beyond the Top 40. This in turn led to rock artists placing greater emphasis on long or experimental album tracks, knowing they could still receive radio airplay.
The progressive rock radio format should not be confused with the progressive rock music genre. While progressive rock music was certainly played on progressive rock stations, so were many other genres of rock as well. Generally everything from early Beatles and early Dylan on forward was fair game. Progressive rock radio was generally the only outlet for fringe rock genres such as space rock and quiet, acoustic-based folk rock and country rock (often played on weekend mornings). Progressive stations were also known for having "turntable hits", songs by obscure artists that did not sell much and were not hits by any conventional measure, but which listeners kept calling up and requesting;Sweet Thursday's "Gilbert Street" was a good example on the East Coast.
The progressive rock radio format grew out of the freeform radio format, and, sharing the key characteristic of disc jockeys having the freedom to play what they chose, has sometimes been referred to as "freeform rock radio" or "freeform progressive radio" or simply "FM rock radio". But as they evolved there were key differences between the freeform and progressive rock formats:
The archetypal successful and influential progressive rock radio station was WNEW-FM in New York in the late 1960s, 1970s, and into the 1980s; Keith Emerson credited it for breaking Emerson, Lake & Palmer into the United States market. Other long-running, large-market examples included WMMR in Philadelphia (credited with helping to break Bruce Springsteen), WMMS in Cleveland, CJOM and WABX in Detroit/Windsor, WXRT in Chicago, KQRS-FM in Minneapolis, KSAN in San Francisco, and KMET in Los Angeles (many of these stations were owned by Metromedia), and college radio stations such as WVBR in Ithaca, New York, WKNC in Raleigh, North Carolina, and WBRU in Providence, Rhode Island. Pioneering progressive rock radio disc jockey and program directors included Scott Muni in New York and Tom Donahue in San Francisco.
Over time (some much faster than others), the large-city progressive rock stations usually lost DJ freedom and adopted the more structured and confined album-oriented rock format in the 1970s or 1980s, and then later the nostalgic classic rock format in the 1980s or 1990s, while the smaller progressive rock stations sometimes turned to the college rock or alternative rock. Where once "progressive rock radio [was] the key media of ascendant rock culture", as writer Nelson George put it, by 1987, musician and author Robert Palmer would write, "The glory days of 'progressive' rock radio - when the disk jockey actually chose the records he played and creatively juxtaposed songs and styles - are long gone." While freeform stations are still around in the 2000s (such as New Jersey's WFMU), there may be no real examples of the specific progressive rock radio format in existence today on the FM dial. XM Satellite Radio's "Deep Tracks" channel might, however, be regarded as a contemporary equivalent. Some of the spirit of progressive rock radio (albeit in a more mellow, "adult" form) can also be found in the Triple-A format.