It's 20 years now since the first hip-hop hit, Rapper's Delight by the Sugar Hill Gang. (The lyrics? "Hip, hop, hip-hip-de-hop" etc and a whole lot of slickly rhymed bragging.)

At that stage, hip-hop was seen as yet another passing fad. But not only is it still very much with us, it has evolved in a myriad of ways. Having begun as the music of black American youth, today 70 per cent of it is bought by white American kids. It is a global phenomenon and, at times, a highly politicised force. And far from being a passing phase, it is frequently hailed as the new rock 'n' roll.

There are certain important similarities between hip-hop and rock 'n' roll: both emerged from black communities areas and, as they were embraced by the white middle-class mainstream, were met with horror by good-clean-fun parents; and both were built on an attitude which decried the oppressive nature of the establishment, an attitude perhaps better known as "f*** you".

But back to the beginning. In one version of the story, hip-hop really began in the early 1970s with a guy called Clive living in the Bronx. His sister Cindy, the story goes, needed some back-to-school money. Clive, originally from Jamaica and in love with reggae, soul and big sound systems, hosted a wild party in the community centre of his block of flats.

Cindy made her pocket money, Clive went on to play more and more parties. In 1973, now known as DJ Kool Herc, he did his first block party. At 18, he was the first DJ to recite rhymes over the instrumental break in a record. He also got a bunch of dancers in to dance during the breaks - break dancers, or as he called them, b-boys.

Next up came Joseph Saddler, who thought Kool Herc was god. He became Grandmaster Flash, the inventor of "scratching" - he, with the Furious Five, created some of the first political hip-hop hits, including The Message and White Lines.

On into the Eighties and the first hip-hop band on MTV, Run DMC, declared themselves the Kings of Rock. That was 1983, and some call it the defining moment, when hip-hop crossed over from black inner-city communities to white suburban living rooms.

Run DMC's next album, Raising Hell, featured a track recorded with heavy-rock veterans Aerosmith; Walk This Way introduced hip-hop to millions of rock fans. It became the most requested video ever on MTV. Before long, the previously rap-free TV station was broadcasting Yo! MTV Raps, which brought hip-hop into American homes on a regular basis.

Public Enemy, in the mid-1980s, deepened the political dimension of hip-hop. Encapsulated in their single, Fight the Power, the Public Enemy ideology was essentially one of black empowerment. Lyrically, Public Enemy helped develop the old rock 'n' roll attitude, "I want to change things for the sake of it", to "I want to change things because they are wrong."

Musically, it was loud, hard and in your face. "I wanted to make music girls hated," said Chuck D of Public Enemy. By 1988, It Takes a Nation of Millions to Hold us Back was in the US top-30 album charts. "It was the first piece of grown-up rap," declared Chuck D.

Hip-hop bands began to emerge right across the United States, each bringing with it a site-specific brand of controversy.

Ice T, for example, from California, dealt with drugs in I'm Your Pusher. "The key to hip-hop is you have to be about where you're from," he said. Niggers With Attitude and Ice Cube introduced a new hard edge to hip-hop, dealing with crime and violence in a completely uncompromising way. "We was just being us . . . We had things to say," Ice Cube said.

From the black ghetto of Compton in Los Angeles, their album Straight Outta Compton got straight to the point, delivering explicit accounts of LA life and gang activity. The likes of F*** the Police, helped put LA on the hip-hop map, expressing the reality of life for an oppressed and abused youth.

However, the FBI was not impressed and made a formal complaint about the record - which lead to loads of publicity and great sales. "For me, rap represents communication," said Ice Cube, but this was not the sort of communication that many parents were terribly keen on.

The 1990s SWUNG into action with the biggest controversy hiphop had yet encountered. Reflecting a Miami lifestyle dedicated to sun, sea and sex, 2 Live Crew popped up with As Nasty As We Wanna Be.

At this stage, hip-hop was not considered the most politically correct of musical forms. For some time, hip-hop bands had been singing about the problems of temptation - but the raunchy performances of semi-naked girls on stage with 2 Live Crew and sexual references in the lyrics which would shock the authors of the Kama Sutra pushed standard rock 'n' roll sexism into the realm of pornography.

Those parents could take no more. Although the album wasn't selling much, it was in suburban homes. The album was attacked, its sale was banned in some places (with a Florida store-owner actually convicted for selling it), the Crew was prosecuted for obscenity and it all became a notorious freedom-of-speech issue. Sales took off.

While sex might have been the preoccupation of some rap bands, police brutality came to the fore with others. In 1991 Ice T released Cop Killer, recorded with a heavy-metal band, Body Count. "It was a potent record against police brutality," he says. At that stage the audience was basically a handful of heavy metal fans, but in April 1992 riots broke out in LA, after police officers who had been caught on video savagely beating Rodney King, a black man they had dragged from his car for no particular reason, were acquitted. Confidence in the LA police plummeted, and Ice T got the blame. "We'd been playing the record for a year and they were able to twist the whole country into believing I'd started the riots," he says.

It was election time in the States. Politicians were competing heavily for media attention, and something like rap, with its increasingly scary connotations, was just the thing to feed into fear and paranoia; or, as Ice T said: "Rap means niggers and whites don't like it."

Dan Quayle, the then vice-president of the US, used a press conference to publicly denounce Cop Killer. It wasn't in fact a rap song, but it was condemned as such. Eventually, Ice T, under intensive legal threats, withdrew the record. According to the presenter of Yo! MTV Raps, the denunciations of Cop Killer were "inherently racist. There were movies with white stars, Bruce Willis, Arnold Schwarzenegger, killing loads of cops, but when it's the voice of young angry black youth, the whole thing became a nightmare."

Into the mid-1990s, and hip-hop was now synonymous with controversy. Gangsta rap, though reflective of the lives of many teenagers living in black ghettoes, appealed to a mass market which revelled in the exploits of gangsters, from Bonnie and Clyde to The Godfather. But it was the celebration of a lifestyle which would see two of gangsta's biggest stars dead before the decade was out - Tupac Shakur and The Notorious B.I.G.

Last year, hip-hop, for the first time ever, outsold what had previously been America's top-selling format, country music.

These days hip-hop is everywhere, from films (Bulworth, any Will Smith film) to fashion and books. Young Kosovar refugees in Macedonian camps shared tapes of home-grown hip-hop which raged against life in prewar Kosovo. MC Solar was the big French wonder, there's been plenty of British hip-hop and we've even had our own hip-hop bands here in Ireland. And it's big in Japan.

These days, while some hip-hop bands revel in material gain, celebrating capitalism, there are still bands with a political agenda; but increasingly, hip-hop bands are more interested in the art of hip-hop, introducing live instruments and a more melodic dimension.

Whatever the time or place, rap has been described as the music of necessity, of finding poetry in the colloquial, beauty in anger, lyricism in violence. It has also been said to represent a change in America's cultural aesthetics - a musical form which has given a previously invisible culture, that of young black America, a voice.