Many people give the credit - or the blame - for the "shock jock" phenomenon to Howard Stern. Stern, who has been offending US listeners for most of two decades, prefers a broader term to describe his importance: "king of all media". Best-selling books, an autobiographical hit movie starring himself (Private Parts) and television programmes give him some justification for the tag, but radio is still where it really happens for Stern, four hours a day, five days a week. From its New York base, his morning show goes out to more than 30 stations around the US.

It all began on WNBC, a popular medium-wave station in New York City. There, "How-Weird" began to develop a sort of "tabloid" radio, full of outrageous, funny stories, sexual suggestiveness and just plain abuse.

The formula hasn't changed much through the years. Last week Stern interviewed a black woman who hates black people, chatted to a woman who works on a phone-sex line and sent his (male) sound engineer walking around the block dressed only in women's underwear after he lost a bet on a baseball game. Early in his career he combined chat and pop music, but nowadays it's all talk - and, of course, advertisements. His favourite activity is probably skewering celebrities. He and his studio team - which includes his foil, Robin Quiver (herself a black woman) - might debate the best celebrity argument, or the best celebrity revelation of a sexual nature. Typically, they'll invent parodies of famous songs to dramatise a celebrity's predicament.

Among his team is Stuttering John, who stammeringly ambushes well-known people with embarrassing questions. Recently, he approached Ted Williams, one of the best batters in baseball history and now an elderly man, and asked: "Did ya ever f-f-f-f-f-fart in the catcher's f-f-f-f-face?"

Not surprisingly, given the high output of sexual innuendo and scatological humour, his biggest fans are teenage boys - and men who still act like teenage boys. These fans get occasional opportunities to show their support: every time Stern's ratings overtake those of a rival presenter in a particular city, he stages a street "funeral" for the rival.

These rivals include a fair few imitators, and many cities where Stern doesn't (yet) broadcast have a Stern soundalike.

Stern expresses contempt for the generation of right-wing talk-show hosts who have occupied the ground cleared by Stern for abuse and vulgarity and used it for directly political ends, with little of Stern's humour. One of them is Rush Limbaugh, who has been called America's "leader of the opposition". He calls Bill Clinton "corrupt, sleazy, dreadful and disastrous".

Imitation of Stern is not just at the fringes of the media. Internationally, the hottest morning-radio format is "the zoo", which usually combines music with humorous sketches, funny extracts from newspapers and studio wisecracks. Some would say it's a cleaned-up version of what Stern and his team have been doing for years.

Stern has often complained, too, that TV's David Letterman is prone to borrowing ideas and personalities heard first on the Howard Stern Show.

But, Stern says, many imitators miss his point - his show, he says, is "sort of about poking fun at me". The self-mockery can get close to self-analysis: Stern's domineering father was a radio engineer who did mock interviews with his son. Excerpts from these old tapes - with his father giving orders: "I told you not to be stupid, you moron . . . Shut up, sit down" - are regularly played on Stern's show.

Stern is well paid for his analysis. Last year he earned more than £10 million, placing him among the highest-earning 100 US entertainers - and way out on top of the radio earnings league. As his boss says, "Every single project he's been involved in has made a ton of money." Obviously, more than just teenagers are listening.

Recently, the Howard Stern Show started to be heard on a station in Montreal, Canada, Stern's first radio foray outside the US. Given that Stern's employer, Infinity Broadcasting, has paid over £1 million in fines to the US Federal Communications Commission for breaches of decency in his programmes, the Canadian authorities are understood to be listening carefully.