In the mid-1970s a group of California kids made skateboarding cool - and created a global industry. Now a documentary narrated by Sean Penn tells their story.

"Today's skateboarders are founders in this sport... It's up to you to see that skateboarding does not become a sport of rebels and radicals. It's a sport for young sportsmen."

- John Severson, publisher/editor, The Quarterly Skateboarder, 1965

There are many fine scenes in Dogtown and Z-Boys, Stacy Peralta's new documentary about skateboarding in the 1970s, but there's one moment that's especially telling. It occurs during a sequence from a 1975 skateboarding event called the Del Mar Nationals, where the Zephyr Competition skateboarding team (aka the Z-Boys), of which Peralta was a member, made their debut with a new, surf-inspired style of skateboarding in front of an unsuspecting crowd.

Today, of course, skateboarding is all but synonymous with teen rebellion, but in 1975 that wasn't yet the case. As Dogtown and Z-Boys reveals, most of the competitors at the Del Mar Nationals were relatively clean-cut, well-behaved types, the "young sportsmen" of John Severson's dreams. They rolled around the freestyle platform that had been set up for the competition looking anything but free or stylish in their tight gym shorts and crisp white slacks. They appeared tentative and over-rehearsed as they performed nose wheelies and other stiff-limbed tricks. One squat, muscular guy even ended his routine with a jaunty double somersault hip roll dismount that looks like Patrick Swayze doing his best Olympic gymnast impression.

Jay Adams, on the other hand, looked nothing like Patrick Swayze, or a gymnast on wheels, or someone who might inspire a skateboarding magazine editor to describe him as a young sportsman. At age 15, he was the youngest Z-Boy and the most charismatic - all sun-bleached, unkempt hair, ratty jeans, and delinquent energy. Instead of performing some rehearsed routine, he improvised. And when he sailed off the edge of the platform in the middle of his routine, it was a perfectly intuitive rejection of the whole idea that skateboarding should involve boundaries, contests, rules and judges. While Dogtown and Z-Boys, which is narrated by skate fan Sean Penn, augments Adams's performance with a Jimi Hendrix song, this isn't really necessary: as Adams slips into a loose crouch, grabs both ends of his board, and hops up and down in a burst of explosive energy speeding across the platform, the implication is clear already. In his charge, a skateboard is no longer a piece of sporting equipment, like a tennis racket. Instead, it's more like an electric guitar, an instrument for aggressive, irreverent, spontaneous self-expression.

And then, after Adams finishes his routine, the moment happens. As he walks away from the platform, the camera follows him for a few seconds, swerving suddenly to capture the reaction of another skateboarder dressed in a nice beige shirt. As the crowd cheers Adams and judges flash scores of nine and 10, the young sportsman's face registers disbelief, then disgust, then a kind of cosmic bewilderment. It's the face of someone who's just realised that a great shift has occurred in his world, and that he's totally unprepared for the new order of things.

It took Jay Adams less than two minutes to show the world a new way to be cool, but his performance had years of history behind it. Like the rest of his Z-Boys teammates, Adams came from a part of southern California that the locals had dubbed Dogtown. Encompassing Venice, Ocean Park, and the southern half of Santa Monica, Dogtown was not a ritzy beachside enclave like Malibu. Indeed, as Skip Engblom, co-owner of the surf shop that sponsored the Z-Boys, explains in Peralta's documentary, their neighbourhood was actually a "seaside slum", characterised by boarded-up storefronts, ratty dives like a transvestite bar called The Pink Elephant and an underground coke-snorting emporium known as the Mirror-Go-Round.

It wasn't exactly the stuff of Beach Boy four-part harmonies. Latino gangs roamed the streets, and junkies shot up underneath the pier at Pacific Ocean Park, an amusement park that was shut down in 1967 and quickly fell into disrepair. But even in its run-down state, Pacific Ocean Park remained a central part of Dogtown culture. Built upon a pier that was 275ft wide and extended hundreds of feet into the ocean, it created three breaks for the locals to surf. But it was also extremely dangerous. "You could get impaled on, like, a fallen roller-coaster track," says former Z-Boy Tony Alva in the documentary.

To become a part of Dogtown's surf culture, you had to be fearless and aggressive. "It was a performance-based environment," recalls Craig Stecyk, an artist who maintained a studio in the back of the Zephyr surf shop. "It was a very, very demanding group of people, because they knew what performance was." But while many elements of the Dogtown ethos existed long before the Del Mar Nationals took place, that ethos might never have spread like it did had it not been for skateboarding.

After all, surfing is governed by definite geographic limitations: only those people who live near the beach can actually do it. But anyone can ride a skateboard, and thus, when the introduction of urethane wheels sparked a second skateboarding boom in the mid-1970s (the first occurred 10 years earlier), conditions were right for spreading the Dogtown style. For several years already, the Z-Boys had been refining their unique approach to skateboarding: instead of performing handstands and nose wheelies, they'd been riding the asphalt banks of local playgrounds as if they were ocean waves. Urethane wheels gave them the ability to skate even faster and turn even harder, but perhaps most importantly, renewed interest in the sport and gave the Z-Boys an audience.

Suddenly, there were kids - hundreds of thousands of kids - who wanted to know more about what could really be done on a skateboard. Very quickly, a magazine called Skateboarder became the new style bible of a generation of teenagers, and in its pages, Craig Stecyk documented and defined the Dogtown ethos in a series of photographs and articles. He wrote about "not reducing skateboarding to a conveniently packaged commodity" or the plastic fun of Disneyland. He chronicled the exploits of Dogtown skaters who scored backstage tickets to Eric Clapton concerts, and exclaimed that the Dogtown boys "were dealing with things too rapid to be observed, the kind that are so quick that they are felt rather than seen".

For budding teenage rebels around the world, it was exhilarating rhetoric, but even better were the photos that he took of the Z-Boys carving the steep banks of the local playgrounds. And when the Dogtown skaters discovered the joys of skating pools, the visual propaganda grew even more potent. Indeed, the images captured by Stecyk, Glen E Friedman (who was just 13 at the time), and a handful of other photographers fixed the essential nature of skateboarding for all time: it was explosive, it was high-risk, and it mandated an aggressive, search-and-destroy attitude that extended far beyond the backyard pools that the Z-Boys used to skate without permission. "[Those photos] showed a lifestyle," says punk icon Henry Rollins in the film. "They showed an attitude. They showed a code."

By 1977, skateboarding had turned into a $400m a year business, and many of the Z-Boys had lucrative sponsorship deals with skateboarding equipment manufacturers. Just a few years later, however, the boom ended. Skateparks shut down because they couldn't get insurance, equipment sales plummeted, and the easy money disappeared.

But while skateboarding's place in the larger culture grew less prominent, the code that Rollins referred to, the code that the Z-Boys incarnated, remained. In the 1980s, skateboarding wasn't a part-time endeavour like singing in the school choir; it was a lifestyle choice, it equipped you with a world-view, and eventually that world-view spilled over into music, fashion, and other high-adrenalin sports like snowboarding and BMX bicycling.

And in the 1990s, a sentiment that Craig Stecyk expressed in a 1979 Skateboarder article finally achieved its full resonance. "Dogtown has gone Uptown," he wrote, in an article that refuted the notion that the Dogtown era had ended. "Or more precisely, Uptown has gone Dogtown."

Conformity has always had its enemies, of course, but in the 1990s, even its most diehard fans decided conformity was a sin rather than a virtue. Christians suddenly felt the need to rock. Businessmen declared themselves anti-authoritarian iconoclasts in the pages of magazines like Wired. Tattoo parlours appeared in malls. Everybody wanted to be a rebel, and be cause skateboarders and their board-sports brethren, tutored in the ways of the Dogtown boys, were such rich repositories of rebellious style and attitude, TV programmers and advertisers enlisted their services. Suddenly, you could not escape the X Games, or totally radical soda commercials, or 1,000 other products that had once been mild but were now somehow eXtreme.

But don't blame the Z-Boys for that. They may have helped popularise a style-is-everything aesthetic that inadvertently terminated in hyper-consumerism, but watch Dogtown and Z-Boys and you can see the purity of their own efforts. Indeed, their cool wasn't purchased at the mall, or achieved through tattoos, piercings, and hair gel, or copied from MTV. It came from their hearts, and from their desire to break new ground - and, 25 years later, in all its surprisingly vivid Super 8 and 16mm glory, it's still a revelation.