As the summer crop of budget-busting films prepares to battle it out on the cinema screens, Film Writer Steve Pratt looks at the phenonmenon of the blockbuster.

Blame it on Hollywood director Steven Spielberg. He started it with his man-eating shark nicknamed Bruce. Before Jaws in 1975, the notion of the summer blockbuster didn't exist.

Quite the opposite: these were the months movie distributors avoided, thinking people were too busy relaxing on holiday, away from school or work, to go to the pictures. Major releases were reserved for other times of the year.

Then along came Jaws, which may have scared away swimmers from the water off the coastal town of Amity but had the opposite effect on cinema attendances. They couldn't get enough of Bruce sinking his teeth into his victims. The modest budget shocker costing $12m became the first motion picture to break the $100m mark.

The gamble on the young, relatively unknown Spielberg's little shark movie had paid off. Spurred on by good reaction to test screenings, the makers decided to go big and put Jaws on 455 screens, a massive number in those days. Usually, movies were given platform releases - shown in a small number of cinemas in the hope that business would build and positive word-of-mouth attract people.

The studios backed the large-scale release of Jaws with an unprecedented amount of TV advertising. The pattern for the summer blockbuster season was laid down. Since then, movies have been getting bigger and bigger, if not necessarily better. Not only budgets increased, so did the number of screens on which films are released. Now blockbusters open on 3,000 screens or more, accompanied by masses of TV advertising and merchandising.

That's why a mean, green anger machine and an out-of-date robot - played, the more cynical maintain, by a screen action hero past his sell-by date - are doing battle at the box-office in coming months. The cast list for 2003's clash of the cinematic titans also includes a swashbuckling pirate with a dodgy English accent, a femme fatale in a figure-hugging wetsuit, a trio of jiggly action girls, and a pensioner adventurer with a Scottish accent.

Perhaps they're more recognisable as The Hulk (who's become less Incredible between small and big screen), Arnold Schwarzenegger, a pirate of the Caribbean, videogame action girl Lara Croft, Charlie's Angels, and Sean Connery.

Between them, they're hoping to entice cinemagoers into multiplexes on both sides of the Atlantic in coming months. We have officially entered the summer blockbuster season. Expect a different one to open each week on your local multiplex screens between now and September.

Big budget, big action, and hopefully big returns. Subject matter is more important than any star. Summer movies are sold on the title and the concept, not the leading actor.

Most US openers will make it across the Atlantic before the summer ends. Only a few, unable to find room in the crowded schedule, will have to wait. Disney's Finding Nemo, from the makers of Toy Story and Monsters Inc, won't appear here until the October half-term holidays, despite opening to record-breaking business in America in June. That leaves the field wide open for Sinbad: Legend Of The Seven Seas to capture the cartoon market over here.

And we'll have to wait a few months for Bad Boys II, which again teams Will Smith and Martin Lawrence. But simultaneous day-and-date release globally is becoming more common, partly to beat illegal copies flooding the market even before a film's opening.

X-Men2 showed the way by opening in the US and 92 other territories within a few days in May. This is a sort of hit-and-run exercise to grab as much cash and attention before the next cinematic juggernaut comes along.

"It's a crazy time for an already illogical business," sums up a top producer. "You have to throw money around because basically you get one weekend for your film. There are no second chances. You're just trying to be heard above the other stuff out there."

The bumper-to-bumper queue of big movies waiting for cinema screens means that studios are often competing against themselves. But Hollywood - and British cinemas following meekly in its wake - can't do without the summer season now, taking advantage of the huge potential audience out there during the holidays.

The strategy helped admissions in the UK and Ireland rise 13 per cent on 2001 to reach 176 million, the highest for 30 years, in 2002. Each weekend around £10m of tickets was sold.

That made it the UK's biggest cinema-going year of the multiplex era, with attendance by the core audience of 12 to 30-year-olds at an all-time high. The UK is the world's third largest cinema market, behind the US and Japan, but moves into second place when video and DVD are added to cinema takings.

You can see how much money is at stake. The average cost of marketing a movie in the US is $30.6m. That added up to the seven major US studios spending a staggering $20.1bn making and marketing 225 films last year.

No wonder studios play it as safe as they possibly can. There's no formula to produce a hit movie, but makers can ensure the odds are in their favour. That means plenty of sequels and remakes, exploiting brands and past hits that are proven winners with audiences worldwide. Originality is not a strong point.

Many titles are follow-ups to $100m-plus first instalments with second helpings of Tomb Raider, Charlie's Angels, and Legally Blonde, alongside third episodes of Terminator, Sky Kids and American Pie (cunningly disguised as American Wedding). The Hulk and The League Of Extraordinary Gentlemen continue the screen's love affair with comic book adaptations. Smaller, more specialised movies can find a place, given careful scheduling and selling. "You can do just fine by going against a monster commercial success, as long as you can appeal to a basic audience," says one box-office expert.

There is little margin for error with budgets hovering close to, and in some cases way over, the $100m mark. A summer movie needs legs: that's the ability to open strongly and sustain big business over ensuing weeks. Up to a third of a film's total takings are reported in the first weekend. Any film that fails to make a splash immediately faces being removed from screens rapidly.

Every film wants to be like Titanic, which became the biggest-grossing motion picture of all time because it enjoyed unprecedented repeat visits by cinemagoers, many of them teenage girls wanting to see Leonardo DiCaprio get wet again and again. Ironically, Titanic wasn't a summer release, yet proved it had real staying power - and not a shark in sight.