Fifty years old this month, the album chart has tracked the history of pop. But only a select few records have actually altered the course of music. To mark the anniversary, Kitty Empire pays tribute to a sublime art form, and our panel of critics argues for 50 albums that caused a revolution. To see the 50, click here

A longside film, the pop album was the defining art form of the 20th century, the soundtrack to vast technological and social change. Once, sets of one-sided 78rpm phonograph discs were kept together in big books, like photographs in an album. The term 'album' was first used specifically in 1909, when Tchaikovsky's Nutcracker Suite was released on four double-sided discs in one package. The first official top 10 round-up of these newfangled musical delivery-modes was issued in Britain on 28 July 1956, making the pop album chart 50 years old this month.

Singles were immediate, ephemeral things. Albums made pondering pop and rock into a valid intellectual pursuit. Friendships were founded, love could blossom, bands could be formed, all from flicking through someone's album collection. Owning certain albums became like shorthand; a manifesto for everything you stood for, and against: the Smiths' Meat is Murder , Public Enemy's It Takes a Nation of Millions to Hold Us Back.

Before lasers replaced needles, albums had sides. They were a game of two halves, building towards an intermission; more than the sum of their constituent songs. At least, the good ones were. Some of them still are, except they can now last 70-plus minutes, over twice as long as their vinyl forebears. Is this bloat, or value for money? The debate rumbles on.

Entire lifestyles built up around albums, smoking dope to albums, having sex to albums. You lent your favourite albums out with trepidation; you ruefully replaced them, on CD, when they didn't come back. Getting hitched paled into insignificance next to merging record collections with your loved one. Getting rid of the doubles made divorce unthinkable. Elastica once sang, of waking: 'Make a cup of tea, put a record on.' That's how generations of hip young (and not so young) people have lived.

But for how much longer? Downloading favours the song, not the album. MP3 players favour personal playlists or shuffling. Listeners are already tiring of keeping company with an artist for an hour or more, as an album meanders beyond mere singles.

The album as we know it might not last another 50 years, maybe not even another 10. But just as artists show groups of paintings in galleries, songs will continue to be written in clumps, connected by theme or time, and presented to a public, just as the Nutcracker Suite once was.

On these pages are 50 clumps of songs, in descending order of importance, that we think caused a sea change in pop music, not always for the good, but without which many bands or entire genres would not exist. They are the sets of songs which have had the greatest lasting influence on music.

It was agonising, having to pick only 50. Why did we include NWA, but not Public Enemy? Probably because their influence was more pervasive. Why Fairport Convention and not The Incredible String Band? Because we had to plump for the single most influential album in British folk rock. And why no Rolling Stones? Because, brilliant though they are, they picked up an established musical idiom and ran with it rather than inventing something entirely new.

Our panel: James Bennett, Kitty Empire, Dave Gelly, Lynsey Hanley, Sean O'Hagan, Elle J Small, Neil Spencer.

· To see the 50, click here.