In New York, the show has had streets named after it and two of its most celebrated stars, Sam Waterston and the late Jerry Orbach, were declared Living Landmarks by the New York Landmarks Conservancy in 2002. If the original Law & Order reaches its 21st season in 2010, which it seems certain to, it will become the longest-running drama in the history of US television, and its creator, Dick Wolf, will no doubt be adding to his estimated $70 million fortune.

It's the yardstick by which all subsequent mainstream US TV dramas have judged themselves, and it could arguably be credited with beginning American television drama's current golden age.

Wolf began his TV career as a writer on the ground-breaking 80s cop show Hill Street Blues. While it might seem a bit tame at this remove, Hill Street Blues was considered gritty and realistic compared to the mild-mannered 70s cop shows -- Kojak, Columbo, Starsky & Hutch -- that had preceded it. Hill Street made reference to the grim reality of inner-city life, and showed the effect that violent crime can have on the people who police it. Working on the series had a big influence on Dick Wolf, but when he came to make his own cop show he decided he could go even further.

First, Hill Street Blues, like practically everything else on American TV at that time, was filmed in studios in Los Angeles. Wolf thought that Law & Order would have a far grittier and more edgy feel it if was shot on location in New York. Second, he had the bright idea of using an hour-long format to show the full journey from crime and arrest to trial and sentence. This would allow him to mix police work with courtroom drama and ask wider questions about the health and workings of American justice.

For his plots, Wolf has regularly been accused of resorting to the true Manhattan crime stories recounted in lurid depth in tabloids such as the New York Post. No story was too grim for the show, which would get into trouble now and then for skating too close to the facts of real cases. Gang rapes in Central Park, dead babies, gangland hits and multiple murders were business as usual for the world-weary detectives of the 29th Precinct and the district attorneys who handled their cases.

The show first aired on September 13, 1990, and its impact was pretty instant. Apart from the grimly realistic nature of its crimes, the show was held together by some memorable recurring characters, and as the series developed Wolf had either the good sense or good fortune to hire two very fine actors, who would become the dramatic bulwarks of the show.

Jerry Orbach, a former song and dance man and veteran character actor who'd appeared in films by Woody Allen and others, actually only appeared as detective Lennie Briscoe in the show's third series, but he became an instant favourite with the public. His hangdog look and knowing wisecracks seemed to sum up the lot of the men and women who clean up New York City's messes, and he would remain on the show until his death in 2005.

Sam Waterston had also worked with Woody Allen, as well as starring in films such as The Killing Fields. He played district attorney Jack McCoy, a brilliant but impatient lawyer who seemed to bring an Old Testament zeal to his prosecuting style. He joined the show in 1994, and McCoy's hang ' em high tendencies ensured that the courtroom segments of the shows were just as entertaining as the action-packed preludes.

Law & Order was made to a rigid format, but was written well enough to get away with it. The show would generally open with a civilian either walking their dog or jogging in the park when they come across the obligatory horrific crime scene. Enter Lennie Briscoe and his younger partner (Chris Noth, Benjamin Bratt and Jesse L Martin all did stints as Jerry Orbach's sidekick), who would shake their heads as they surveyed the corpse, before Lennie cracked wise with some graveyard humour.

The detectives would proceed to solve the crime, but usually play fast and loose with the law while extracting the necessary evidence from obtuse villains, which would make Jack McCoy's job all the harder when the time came for prosecution. Judges came in all shapes and sizes, and often seemed more interested in their careers than the rights and wrongs of the cases before them. And the show's scripts regularly threw up the kind of moral dilemmas that test a legal system to its limit.

It was a simple format but a brilliant one, and in the late 90s Wolf felt his show was established enough to spawn a few spin-offs. Law & Order: Special Victims Unit appeared in the autumn of 1999 and made the original show look like an episode of Garda Patrol, for the cops in this series dealt with sex crimes and assaults of unimaginable cruelty, which tended to have an attritional effect on the mental health of the boys and girls of the Special Victims Unit.

Chief among these were detectives Elliot Stabler (Christopher Meloni) and Olivia Benson (Mariska Hargitay), who seemed suspiciously close but never actually got it on. Among their associates were Munch (Richard Belzer) and Tutuola (played by former rapper Ice-T), and Law & Order: SVU delved further into its recurring characters' private lives than the original show had. Hargitay, the bombshell daughter of 50s starlet Jayne Mansfield, has become a sex symbol and major star on the back of the show, which is now in its 11th season.

It was followed in 2001 by Law & Order: Criminal Intent, a show that concentrated on the work of the Major Case Squad, a group of detectives who dealt with high-profile crimes and murders involving celebrities and public officials. Its star was Vincent D'Onofrio, the talented but unpredictable film actor whose portrayal of detective Robert Goren is eye-catching to say the least. In pursuit of his felons, D'Onofrio stands with his head to one side, pulls bizarre faces and pauses for lengthy periods before exploding into tirades of sometimes impenetrable dialogue.

It's an eccentric characterisation that people tend to love or hate, and it will be interesting to see how Jeff Goldblum, who's no stranger to the odd thespian tic himself, will compete with this unique technique.

Vincent D'Onofrio is planning to leave Criminal Intent next year, but meanwhile it, Law & Order: SVU and the original of the series plough on with no sign of fading. Actors of the calibre of Robin Williams, Samuel L Jackson, William H Macy, Allison Janney, James Earl Jones, Jennifer Garner and Kathleen Turner have cameoed as lawyers and criminals, while public figures such as Rudy Giuliani and current New York mayor Michael Bloomberg have also been happy to appear in the franchise that's long since become a beloved Manhattan institution. In fact, this year, Mayor Bloomberg honoured the show's 20th anniversary by launching an annual 'Law & Order Day'.

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The new series of Law & Order: Criminal Intent screens on Wednesdays at 9.30pm on RTE2