I'VE had this conversation, or one like it, so many times this year that I've lost count. Other person (on learning I write about television): "So, is there anything good on now?"

Me: "Well, yes, did you see The Line of Beauty/Jane Eyre/Peep Show/Extras. . . and of course you can't go far wrong with Curb Your Enthusiasm. . . " Other (curbing their enthusiasm): "Yes, but do you know what I watch?"

Me (with a creeping sense of inevitability): "Er, no."

Other: "CSI. I love it. CSI, CSI: Miami and now CSI: NY. But I think CSI: Miami is my favourite. In fact, I'm a bit addicted. . ."

These CSI addicts don't have the understandably haunted air of West Wing and Sopranos junkies, worried that their fix may end with a brutally swift network executive decision. CSI fans announce their habit with the quiet satisfaction of people who know what they like and like what they know. Their beloved shows don't attract the critical love-ins of 24 or Lost.

It's a non-vocal minority. And anyway, CSI fans would be right in not thinking of themselves as something special, as part of a cult.

Far from it. In fact, they are engaging with the most popular TV shows on the planet.

The similar but distinctive series that make up this franchise - CSI: Crime Scene Investigation, CSI: Miami and CSI: NY - are syndiI cated in 200 countries to a global audience of two billion. You have to go back as far as Baywatch for anything even remotely close to that kind of penetration. About 60 million Americans tune into the three shows every week, and it's single-handedly turned round the fortunes of its maker CBS, leaving the oncemighty NBC - purveyor of ER and Friends - looking to its laurels. ABC, which turned down CSI because they reckoned it would be too complicated for the audience, were left feeling. . . well, you can imagine. It's like being the man who passed on The Beatles.

In the US, CSI dwarfs the likes of Desperate Housewives and Lost, the recent critical darlings. A little explication is probably in order.

The original CSI: Crime Scene Investigation began in 2000 and followed the high-minded, resolutely logical Gil Grissom (played by the moderately successful film actor William Petersen), a father figure to the dedicated professionals of the Las Vegas Police Department Crime Lab. CSI eschewed the street-slogging, suspect-berating style of its forerunners NYPD Blue and Hill Street Blues for forensicled investigation - not so much a whodunnit as a howdunnit. Grissom is more likely to pull an electronic spectrometer than a gun.

Viewers immediately took to the way these geeks got to solve crimes by, according to the show's catchphrase, "following the evidence".

The critics agreed and CSI: Crime Scene Investigation alone has won six Golden Globes and 24 Emmys.

In 2002, the CSI franchise began to develop, setting up shop in Michael Mann's old stomping ground, Miami. Immediately, the makers had another hit show on their hands.

CSI: Miami was followed in 2004 by CSI: NY, viewers happily adding a third show to their weekly diet. Three hit shows on the trot - CSI has come an awfully long way since a struggling scriptwriter collecting tickets on the Las Vegas trams, Anthony Zuiker, first dreamt up the idea.

A large, bald and ebullient man with clipped beard and wrap-around tinted spectacles, Zuiker wouldn't be out of place at a sciencefiction convention. Inspired by watching the OJ Simpson trial unfold on TV, Zuiker managed to persuade the real Vegas CSIs - then called Field Services but now, thanks to the show, actually called CSI - to allow him to shadow them for five weeks. This is a favour that would probably not now be granted, thanks to the popularity of his own creation. Everybody, it seems, from Britney Spears (reportedly wanting to swap her pop career for forensics) to eager school-leavers with no science education whatsoever, wants to get in on the crime scene investigation act.

Zuiker took his idea to Jerry Bruckheimer, the Hollywood producer behind such action blockbusters as Beverly Hills Cop, Top Gun, Con Air and Pearl Harbor. Bruckheimer recalled Zuiker's pitch ("You walk in and there's a dead lady on the floor, a plant knocked over and a pink elephant in the back yard - what happened?"), but above all he remembered his enthusiasm. A deal was struck.

"We started in 2000 and it was a success, but our ratings really shot up after the 11 September attacks, " Zuiker says. "People were rushing to us for their comfort food. There was a sense of justice in CSI - it helped to know that there were people like our characters out there helping to solve crimes. And, of course, 9/11 was the world's largest crime scene."

There have been forensic crime dramas before - notably Silent Witness, in which Amanda Burton spouted medical jargon over mortuary slabs. What makes CSI so different?

You could start with the special effects. If a picture is worth a thousand words, CSI's microscopic tunnelling along gunshot wounds, or down the throat of a "vic" (that's CSI shorthand for victim; you soon pick up the jargon) are worth a hundred weighty expositions by characters in white coats. Dead babies, severed limbs, you name it - all are grist to the CSImill.

The only place they're unwilling to penetrate, for reasons of taste, is the human backside.

These cleverly filmed inserts not only bring to life what happens when a bullet rips through skin, bone and organ, and illustrate the significance of blood splatter or ripped fibres; they also move the story along as well as any conventional cop show car chase or shoot-out. The makers use tiny cameras and lenses, or else oversized props, to achieve their effects. CSI is never about showing off, and these bravura sequences play their part and swiftly exit the stage. But they leave the viewer with no doubt that they're in the presence of a TV series that wants to be bigger and bolder.

It's all about production values. Each episode of the three CSI shows costs $3m to make and takes eight days to shoot - and a season's filming lasts 10 months. A lot of that money is up there on the screen, reminding you the kingpin here is Bruckheimer, a man versed in Hollywood blockbusters and providing bang for your buck.

TV, he's saying, need not look cramped and second fiddle to the big screen. "It's all about doing 'feature television', " Zuiker says. "We do 45-minute Bruckheimer-esque movies every week, with a level of editing and sound and pace unlike anything in motion pictures." CSI is a hybrid, and as such the more adventurous big-screen directors have been attracted to the show. The director of last season's doubleepisode finale of CSI: Crime Scene Investigation was Quentin Tarantino.

But if the production values are new, then the dramas also depend on some old-fashioned storytelling virtues - like the heroic central protagonist. Each of the CSI teams is built around a male authority figure, starting out with CSI: Crime Scene Investigation's Grissom. Part Sherlock Holmes and part Rupert Giles from Buffy the Vampire Slayer, Grissom is a monk-like scientist given to lofty statements which, surprisingly or not, don't stop him being a bit of a hit with female viewers.

Our next hero was David Caruso, the former NYPD Blue star turned flop movie lead who had more or less given up acting to concentrate on a chain of upmarket clothing and furniture stores in Miami. The right man in the right place, Caruso may be a weedy-looking redhead, but the actor has the most macho murmur this side of Clint Eastwood. Famously given to removing his sunglasses to emphasise a point and breaking up his sentences with dramatic pauses, his character even shares a first name with England's most heroic naval officer. Lieutenant Horatio Caine is wide open to parody, but he manages to remain on the heroic side of laughable.

And finally there is Gary Sinise in CSI: NY, the most modern and complex and the least archetypal of the trio, his greyness suiting the post-9/11 Manhattan location. In fact, the writers gave Sinise's character, Detective Mac Taylor, a shameless back-story; a wife who died in the twin towers. Man and the city as one.

Of course there's the common female refrain: men only tune in, not for the forensics, but the foxes. They do have a point. The crime units of Vegas, Miami and New York do happen to be stocked with more than their fair share of beauties. For Prime Suspect, Helen Mirren talked about how, when she first researched her Jane Tennison character, a lady copper told her that policewomen invariably wore their hair cut short so that criminals couldn't grab hold of their tresses in a fight. The females in CSI look like they've just had a full grooming session with Nicky Clarke.

But when it comes to glamour and CSI, the heroic males and leonine females take a back seat to the real stars of the shows - three of the most distinctive conurbations in America.

"We cast a great city, " says the executive producer Anne Donohue. "We let each city inform us and then we produce to that."

So it's no accident that CSI: Crime Scene Investigation takes place largely at night, in that neon-lit sin city, Las Vegas. CSI: Miami, by contrast, unfolds mostly in daytime, taking advantage of the hot colours, bright clothes and eternal sunshine. Each opening sequence is an aerial tour of its show's respective city, to pounding theme music from The Who ('Who Are You' in CSI, 'Won't Get Fooled Again' in CSI: Miami, and 'Baba O'Riley' in CSI: NY). No wonder Pete Townshend and the boys are currently on a sell-out tour of the States.

Bruckheimer encapsulates the show's appeal thus: "A combination of great writing and great casts and it's a show that's intriguing each week."

What he probably wouldn't disagree with is that CSI is not creatively cutting-edge TV in the manner of darker, grittier crime shows such as Homicide: Life on the Street or the Baltimore-based drugs drama The Wire. And, although care has been taken to differentiate the characters, they are not nearly as distinctive as the cops of NYPD Blue, or those of the most characterful crime series of them all, Steven Bochco's seminal Hill Street Blues, with its wonderful gallery of undercover nutcases, weathered cops and unscrupulous detectives.

The CSI shows are also heavily formulaic, but they are always suspenseful, and there's invariably an interesting new angle. They apparently scour the tabloid press for inspiration, and talk endlessly to real-life crime investigators. Zuiker says: "If it's happened once, and it's cool, then it's in the show."

Such an endless search for novelty - and the many ways these criminologists find to solve cases (always within the hour) - has not been without repercussions in real life. The American public now demands the highest standard of forensic investigation, which is unreasonable because Zuiker's fictional CSI labs are the best-equipped in the US, and his show has some of the most experienced criminologists working on it. "Forensic experts prefer to work on the show, " Zuiker says. "It's better pay, for one thing."

In other words, life is struggling to imitate art. Juries have been acquitting accused murderers because they haven't seen the same standard of evidence as they saw on CSI the night before - including last year's acquittal of the actor Robert Blake for the murder of his wife, despite the testimonies of 70 witnesses.

And it's become almost standard for judges to lecture juries at the start of a trial that "they are not watching an episode of CSI".

Meanwhile, criminals are getting a crash course in crime scene destruction. Cases where crime scenes have been doused in bleach (bad news for the forensics folk, apparently) have gone from extremely rare to commonplace. On the positive side, there has been a huge increase in interest in forensic science ("From about five applications for forensics jobs a year to more than 5,000, " Zuiker says) and some people claim to have detected - although this is unquantifiable - a surge of interest in science generally. Zuiker calls it the "CSI effect" and is hugely proud of his creation. "For the first time in American history, you're not allowed to fool the jury any more."

That's open to debate. But as the search for certainties pushes crime detection ever closer to Philip K Dick's imagined world of Minority Report, where people are accused of murders they haven't yet committed, does it finally signal, exactly 120 years after Arthur Conan Doyle wrote his first Sherlock Holmes story, the death of the classic murder mystery story?