At the end of the 500th episode of The Simpsons, which screens in Australia next week, there's a black and white title card that reads: "Thanks for 500 episodes. All we ask is that you go out and get some fresh air before logging onto the internet to say how much this sucked."

It is a strangely sour note with which to mark such a major landmark for one of the most successful and culturally influential shows in television history.

First screened in the Unites States in 1989, on what was then the fledgling Fox Network, and in Australia in 1991 on the TEN network, The Simpsons is the longest running TV sitcom. Over the past 22 years it has netted 27 Primetime Emmy Awards, earned billions of dollars for Fox, and been named the best television series of the 20th century by Time magazine.

Numerous words and catch-phrases from the program have more or less entered the language, including: avoision, bathiola, capdabbler, cheese-eating surrender monkeys, cromulent, embiggen, diddly, d'oh!, dorkus molorkus, eat my shorts, meh, neighborino, okely dokely, smell ya later, snitchy, yoink, and my own particular favourite, craptacular.

The Simpsons will be remembered above all for its glorious cast of secondary characters - and for the inner lives they often struggle to express. As in the novels of Charles Dickens it is these secondary characters who often stay most powerfully in the memory: the evil billionaire C. Montgomery Burns pining for his lost teddy-bear, Bobo, in a plotline based on Citizen Kane; or his groveling factotum Mr Smithers, struggling to express his love for Mr Burns during a meltdown at the nuclear plant:

Mr Burns: Oh Smithers, I guess there's nothing left to do but kiss my sorry arse goodbye.

Smithers: May I, sir?

Image courtesy of Network Ten

Sideshow Bob

Bart's single, lonely and sighing schoolteacher, Edna Krabappel, surely represents a significant contemporary female archetype. And his nerdy and envious sidekick, Milhouse, is a perfect comic creation. On one occasion when Bart discovers that the school once had an even bigger prankster than himself, Bart, Milhouse responds:

"Wow! Imagine his sidekick.Man, if he lost that giant inhaler, he'd really be in trouble with his parents."

The psychological implication is that Bart's pranks make Milhouse feel physically larger and stronger.

As with Dickens, The Simpsons seems endlessly interested in outcasts, criminals, murderers and misfits. Perhaps the darkest and most disturbing (because he does not represent pure evil) is the ugly, socially awkward, violent and sometimes suicidal bar-keeper, Moe. But my personal favourite is the broken-down television clown, Krusty ("I work like I drink: alone, or with a monkey watching"). A clown who hates children, Krusty only stays in show business because he needs the money he gets from cranking out cheesy merchandise. Estranged from his rabbi father, Krusty is addicted to gambling, booze, cigarettes and sex chat-lines, and has 15 broken marriages behind him (the 14th, to Eartha Kitt, lasted just a few hours).

Krusty is Homer's doppelgänger, drawn to look exactly like him. In many ways he is the anti-Homer. Both live for pleasure, but where Homer's excesses only serve to make him fat, all Krusty's excesses are visited on his haggard, worn-out body.

There is a wonderful moment in an episode called "Homie the Clown" when Homer goes to clown school in order to become a Krusty double and franchisee. Almost immediately he finds himself changing into a dark, angry clown too. The life of a professional clown turns out to be a thankless one:

Image courtesy of Network Ten

The Simpson family

Homer: Aw, being a clown sucks! You get kicked by kids, bit by dogs, and admired by the elderly. Who am I clowning? I have no business being a clown. I'm leaving the clowning business to all the other clowns in the clowning business!

The joke is that if Krusty is a dark clown, Homer is the perfect happy clown - perhaps the greatest clown of all time, and certainly the greatest cartoon character since Daffy Duck, an American Everyman who personifies all that is most mediocre, lazy, anti-intellectual in modern America.

What sustains any good television comedy is the quality of the writing. But in The Simpsons, prior even to the writing, there's the importance of the voices. There is something about cartoons that makes us confront the mystery of human personality, and it is said that from the very beginning it was the ambition of producer James L. Brooks that these cartoon characters had to feel like real people. Accordingly, he insisted on employing real comic actors. The result? How many of these characters exist in our imaginations as a voice before anything else: the wheedling tones of Mr Burns that voice-actor Harry Shearer reportedly based, in part, on President Ronald Reagan; comedian Kelsey Grammer's rich baritone for the character of sideshow Bob, singing his way through HMS Pinafore; Hank Azaria's Apu and Comic Book Guy. Above everything else, The Simpsons will continue to be remembered for the dazzling virtuosity of its vocal acting.

It is hard to say just what The Simpsons represents in cultural terms, so ubiquitous and global is its reach. My guess is that it will come to be seen not merely as an exceptional pop culture artifact, but as a key to understanding how postmodern television culture actually works and how to read it.

Looking back, it is hard to believe that when the program first started, conservatives in the United States regarded The Simpsons with horror as politically subversive and a threat to traditional family values. Campaigning in 1992 during the election that he would eventually lose to Bill Clinton, President George H.W. Bush famously said he wanted American families to be "a lot more like the Waltons and a lot less like the Simpsons".

Although The Simpsons is liberal in its politics and satirizes American consumerism and anti-intellectualism, it usually ends up endorsing the status quo. Professor Paul A. Cantor of the University of Virginia wrote an award-winning essay on this subject, "The Simpsons: Atomistic Politics and the Nuclear Family" (Political Theory, December 1999), in which he argues that The Simpsons affirms both the nuclear family and small-town American values. "What makes The Simpsons so interesting is the way it combines traditionalism with anti-traditionalism. It continually makes fun of the traditional American family. But it continually offers an enduring image of the nuclear family in the very act of satirizing it…. For all its postmodern hipness, The Simpsons is profoundly anachronistic in the way it harks back to an earlier age when Americans felt more in contact with their governing institutions and family life was solidly anchored in a larger but still local community," he writes.

Similarly, although it continually satirizes institutions such as the school, local government, the police and the American corporation, because it so self-consciously recreates the world of 1950s sitcoms like I Love Lucy, Leave It to Beaver and Father Knows Best, it ends up celebrating old-fashioned small-town America. As Cantor puts it, "The Simpsons shows the family as part of a larger community and in effect affirms the kind of community that can sustain the family. That is at one and the same time the secret of the show's popularity with the American public and the most interesting political statement it has to make."