Progressive Rock (or known as Prog Rock) has a wide span of influence in many Classic Rock bands.
It could all be traced to the Beatles "Sgt. Pepper's Lonely Hearts Club Band" because of its free-form and avant-gard during the height of 1967 and the Summer of Love. Some may even consider Mod as the root toward Progressive Rock. The first string of Progressive Rock Bands were The Moody Blues, Pink Floyd, Traffic, Procol Harum, and The Move. The first true Progressive-Rock song was Procol Harum's 'A Whiter Shade of Pale' which used Bach's 'Air on a G String'. The first true Progressive Rock album was The Moody Blues' "Days of Future Passed" which came in late 1967 and between 1964 and 1966, The Moody Blues were more blusier and when Clint Warwick and Denny Laine (Who later would join Wings) left, they became more moodier.
In 1968, The Pretty Things released "S.F. Sorrow" and late that year, the Who released their rock-opera "Tommy". It seemed that Progressive Rock even came from bands and musicians that usually aren't considered Progressive Rock such as Jimi Hendrix, Elton John, Chicago, Stevie Wonder, Led Zepplin, and Steve Miller to name a few. But in 1969, other Progressive-Rock artists formed such as King Crimson, Yes, Jethro Tull, Genesis, Camel, Soft Machine, Gong, and Gentle Giant.
Between 1972 and 1973, Progressive Rock came in full force with many albums in the top-10 such as Moody Blues' "Seventh Sojurn", Procol Harum's "Live in Concert with the Edmonton Symphony Orchestra", Yes' "Fragile", Jethro Tull's "Thick as a Brick", Todd Rundgren's "A Wizard/A True Star", and ELP's "Brain Salad Surgery". Emerson, Lake, and Palmer were formed by three musicians from three different bands that united at The Isle of Wight Festival in 1970 by Keith Emerson of The Nice, Greg Lake of King Crimson, and Carl Palmer of The Crazy World of Arthur Brown. And Jimi Hendrix was about to join calling the Band HELP, but due to a drug overdose by Jimi Hendrix, it became a three-piece band.
In North America, Progressive Rock didn't have a stronghold until the 70's when bands such as Styx, Kansas, Klaatu, Frank Zappa, Utopia, and Rush formed at the time. The main roots of American Progressive Rock came from bands such as The Left Banke, and Strawberry Alarm Clock. In the mid-70's, bands such as Queen, Roxy Music, 10cc, ELO (Which came from The Move), and Supertramp were formed, but at the time Disco and Punk pushed many bands such as ELP, Jethro Tull, Procol Harum, and Traffic into disbandment.
In the 80's, many New Wave artists took the Progressive Rock ideas into many of their songs. Bands like Talking Heads, The Buggles, Devo, Gary Numan, Europe, Thomas Dolby, and Squeeze used synthesizers in their music and forming off-the-wall antics. And bands such as Pink Floyd, Yes, and The Moody Blues survived the 80's. Another band called Asia had members from Yes, ELP, UK, King Crimson, and Uriah Heep which even had a hit called 'Heat of the Moment'.
Now, Progressive Rock is coming back with bands such as Marllion, Glass Hammer, Spock's Beard, IQ, Tool, Porcupine Tree, and The Divine Comedy use the same progressive movements as their prodecessors. And bands such as Procol Harum, Jethro Tull, Moody Blues, Pink Floyd, and Genesis still tour. It seems that Progressive Rock will never go away.