(FORTUNE Magazine) – When was it, exactly, that the American teenage girl ceased to be demure? More than likely it was in the autumn of 1944. That was when Seventeen magazine, the original journal of pubescence, was launched. It was also when 30,000 bobbysoxers swarmed outside Manhattan's Paramount Theater one morning to see Frank Sinatra. When they were told the show was sold out, the girls rioted. They mobbed the streets, broke shop windows, and destroyed the Paramount ticket booth. Hundreds of police rushed in to control the frenzy, and the next day's headlines confirmed (with alarm) the existence of something new: girl power.

Elvis knew its importance, and so did the Beatles. So did moviemakers, publishers, and manufacturers of saddle shoes, bell-bottoms, Afro combs, and pimple creams. The teenage girl of the postwar era was boisterous, free-spending, suggestible--a budding consumer, doted on by marketers hoping to line up her allegiance for life.

But then a funny thing happened: The adolescent girl became invisible. Once the baby-boomers began hurtling toward middle age and having babies of their own, the teens who came after them--Generation X--didn't seem to count much anymore. They didn't wear makeup, favored old flannel shirts, and embraced antiheroes like Nirvana's Kurt Cobain. Maybe teenagers had become independent and inscrutable. Maybe Xers were making a statement about the materialism of their elders. Or maybe marketers were just so fixated on the 78 million boomers that they ignored--or misunderstood--the 45 million Xers born between 1965 and 1977. ("News flash!" screamed Time magazine in a June 1997 cover story about how--golly!--Xers weren't so different after all. "They are confident, savvy, and, [a new] survey concludes with a measure of relief, materialistic.")

Whatever. The teen girl is back--with a vengeance. The surprise hit film of 1997 was Scream, a tiny, $14 million Miramax horror flick aimed at teenagers. It grossed $103 million at the domestic box office and has since spawned a revival of slasher movies (Scream 2 will hit theaters by Christmas).

Plenty of boys saw Scream. But the girls --well, put it this way: One 17-year-old girl I spoke to has seen Scream seven times. It is the teenage girl who will line up for hours, even in a heavy rain, to see your movie on opening night. If she likes it, she and her friends and her friends' friends will return again and again. If she's moved by your CD, she'll buy your licensed T-shirt, book bag, candy bar, screensaver, lip gloss, cola, umbrella....

After declining for 15 years, the number of teens is now finally on the rise. And their ranks are growing at a faster rate than the overall U.S. population. By 2006, teens will number nearly 30 million, the highest level since 1976. All told, this generation (call them what you will--echo boomers, Generation Y, generation wired, the digital generation, millennials--they're kids born since 1978) totals 77 million.

Retailers are falling in love again. Kurt Salmon Associates' 1997 Annual Consumer Survey reports that only 55% of Americans ages 21 to 62 "enjoy shopping"; that's down from 58% in 1993. In sharp contrast, 88% of girls between 13 and 17 say they just "love to shop." No surprise, then, that according to the International Council of Shopping Centers, teens make nearly 40% more trips to the mall than other shoppers do.

And they're not just window-shopping. Teens will spend $84 billion of their own money this year, predicts Teenage Research Unlimited, a market research firm based in Northbrook, Ill. Where do they come by such a sum? The Bureau of Labor Statistics reports that 41% of teens ages 16 to 18 work full- or part-time. If you add in jobs not recorded by the government, like babysitting, three-quarters of teens earn at least some money from working, says TRU.

High school senior Patricia LaBarre--the girl who saw Scream seven times--works 25 hours a week selling clothes at Express. She spends her money as quickly as she earns it. One Saturday afternoon in mid-October, Patricia emerged from a clothing store at New Jersey's Garden State Plaza mall carrying a new pair of flared corduroy pants, a blue V-neck sweater, and a white tank top. Total tab: $60.50. That's almost half her weekly wage.

Girls spend lots of their parents' money too. Jennifer Esner, 12, is a fan of Steve Madden, the trendiest shoe brand around. In late October she persuaded her mom, Camille, to buy her Madden's new $99 Cynthia boots. A week earlier she got a pair of Madden's Jason shoes ($79). In late August she got Madden's Skate sneakers ($49). Is it any wonder Madden's stock has climbed from 3 1/4 in April to a recent $7 1/4?

Boys spend money too. But it's girls who move markets. "A guy will buy one, maybe two, [surf] board shorts each summer--and if there are not too many holes in them, he'll probably wear them again next summer. But a girl goes through four or five pairs a summer, and there's no way she'll wear the same ones again next year," says Randy Hild, head of Roxy, the women's division of Quiksilver Inc., the largest maker of surfwear.

Since Roxy was launched in 1993, girlwear has come to account for 20% of Quiksilver's $220 million in sales. By 2000, maybe half of Quiksilver's business will come from girls. The 28-year-old firm was barely breaking even before the launch of Roxy; now it's booming. The stock has shot up 50% to the $30 range in the past year--despite the market correction. That's girl power.

Pacific Sunwear of California Inc., a retailer with 263 stores in 37 states, sells those hot skateboard and surfer looks, like seriously baggy pants, extra-large T-shirts, knitted caps, and wrap sunglasses. It's trendy stuff, but like Quiksilver, PacSun was lagging until it got hip to girl power in late 1995. Now 18% of revenue is generated by girls, and PacSun's stock has doubled since January.

Now everyone wants a piece of the action. For years, Seventeen, YM, and Teen have had the girl-magazine market to themselves. This year there are two new magazines on newsstands: Jump, from Weider Publications, and Twist, from Bauer Publishing. In January, Time Warner (parent of FORTUNE's publisher) will enter the market with Teen People--expecting 85% of readers to be girls.

But nobody has cracked the teen code better than Delia's Inc., a catalogue company for girls with projected 1997 sales of $70 million. Since Delia's went public a year ago, its stock price has doubled to around $22, a dizzying 51 times estimated 1997 earnings. Today's teen girls can't get enough of Delia's shiny purple and silver "avalanche" pants ($44), shrunken baby-T's that read GREED GIRL ($24), and body glitter ($5). Delia's receives between 3,000 and 5,000 unsolicited requests a day for catalogues. The mailing list itself is a glittering gold mine: So far it includes 2.6 million names and addresses.

"It's a gorgeous, gorgeous business," purrs 32-year-old Chief Executive Stephen Kahn. He launched Delia's in his New York living room in September 1993 with his former Yale roommate, Christopher Edgar. To get Delia's started, Kahn contributed $100,000 saved from a previous job at PaineWebber. Today Kahn's 57% stake is worth $163 million.

Delia's is such an obvious idea that you can't believe you didn't think of it first. The marketing concept is this: Today's average 14-year-old girl in Des Moines is just as hip to what's hot as the 14-year-old in suburban Los Angeles (thank you, MTV). She, too, wants shiny avalanche pants and baby-T's, but she's stuck in the backwoods with nowhere to shop but her local Wal-Mart. Delia's body glitter, like Dorothy's red shoes, transports her from the farm to Melrose Avenue.

As if to prove this point to its advertisers, each issue of Seventeen magazine profiles a typical high school in a typical American town or city. In August it was Hellgate High School in Missoula, Mont., where 15-year-old Anya, photographed in lace-up vinyl shin-high boots, declared, "I like wearing clothes that no one else would dare to wear."

As Missoula meets SoHo, Delia's doubles are turning up everywhere. Zoe, another catalogue for girls, was launched in August by Fulcrum Direct, a $37 million (1996 sales) direct-marketing company in Rio Rancho, N.M. In January yet another girls' catalogue is due from Wet Seal Inc., the biggest, trendiest retailer of girls' apparel, with 370 stores in malls across America.

And then there's Airshop. Last year Dominique Camacho, 29, started Airshop, a catalogue for hipsters like Missoula's Anya. By last September, the buzz was loud enough to attract two bankers from J.P. Morgan to Airshop's unmarked headquarters in a nondescript rowhouse in Manhattan's East Village. Since then Camacho has sold 40% of her tiny company to apparel manufacturer Jalate Ltd. (without help from J.P. Morgan).

What sets Airshop apart is attitude. Delia's and Zoe's cover girls are fresh-faced blondes; a recent Airshop cover featured Hafsa Ibrahim, who's part Haitian, part Sicilian, and part Mongolian--a girl with pierced eyebrows and a grin as big as the Ritz. Airshop's tag line: "The tastiest clothes for the coolest young Bettys out there!"

Airshop knows about girl power. Its catalogue runs pictures and poems sent in by its creative customers. It publishes mini-profiles of teenage girls---girls who are kicking butt in male-dominated fields (the spring catalogue will profile a professional Website designer, a techno girl, age 14). Next to a review of the British band the Chemical Brothers is a girl modeling the "Japani Grrrl Tee" (Color: blue. Sizes: S, M, L. Price: $26).

Is Airshop a hip, shoestring literary magazine or a catalogue for adolescent consumers? It's both. If you want to sell to the girl-power crowd, you have to pretend that they're running things, that they're in charge.

"Weaned on computers, kids today are dramatically different. They expect to have much more of an influence on their medium," explains Geraldine Laybourne, who heads the Disney/ABC cable networks. "We programmers will not be able to push stuff on kids as easily. We will have to get clever at making them feel like they're in control."

The big company that understands girl power best is Nike Inc., whose Website for teens is called Play Like a Girl. On the site is an essay by Karli, a 15-year-old girl in power: "When my mom was younger, if someone said you play (or run or throw) like a girl, it was considered an insult. But I think it's like a compliment now.... So when someone says I play like a girl, I ask which one? Lisa Leslie or Dawn Staley or Mia Hamm?" (FYI: These women are professional athletes. Get to know them.)

Reebok targeted women before Nike did. But Reebok in the 1980s was all about fitness (not sports), exercise (not competition), jumping up and down in pink-and-black Spandex. It was wimpy. It was something your mother did to keep from sagging. Nike girls, on the other hand, are tough. They play pickup ball with the boys in the 'hood. Which helps explain why, since 1993, Nike's sales to girls (and women too) have grown from less than 15% of total U.S. sales to about 25% today.

Since Title IX (the 1972 civil rights statute barring gender discrimination in education) came into effect, the number of high school girls who participate in team sports has gone from 294,000 to 2.4 million. That includes girls who play football, hockey, and basketball--in other words, girls who kick butt. Along with the growing number of powerful women on television, in boardrooms, and in courtrooms, the change has had a huge impact. "Every 14-year-old girl today really believes she can become President, that she can run Exxon," says Delia's Steve Kahn.

Maybe she can, maybe she can't. The point is that girl power is a state of mind that smart marketers are targeting. Being Hispanic, being young, being black, being a woman--these are apparently not barriers. "When I was a teen, I knew I was the wrong answer; I knew people didn't want to hear from a young black woman," says Terri Gardner, the 41-year-old chief executive of Soft Sheen Products Inc., the country's biggest maker of ethnic hair-care products. "These girls don't have that in their head. They're going to attack the world. It's very refreshing, very empowering."

This is a generation that has been catered to since birth: Baby Gap, Nickelodeon, Sports Illustrated for Kids. They're special. They're entitled. They've earned the Steve Madden boots, the funky baby T's, the body glitter.

In the most recent quarterly survey from Teenage Research Unlimited, 73% of teens agreed with the statement: "Things are really going well for me. I've worked hard to get where I am and feel I'll always be successful." You can spot the perkiness in the sorts of music girls between 12 and 17 are buying: In the first eight months of 1997, 21% of the money they spent on music went toward "alternative" bands like Smashing Pumpkins--down from 27% just a year earlier. Meanwhile, saccharine pop bands like Hanson (three home-schooled brothers from Oklahoma) or the Spice Girls (five saucy girls from London) now account for 7% of spending, up from just 2% a year earlier.

Alexandra Fondren, an articulate, self-possessed 13-year-old from Franklin Lakes, N.J., captures the prevailing mood pretty nicely. Her favorite TV ads are Wrigley's peppy Doublemint (double fun) gum series. Her bedroom wall is papered with 16 advertisements for milk--celebrities with milk mustaches. She "loves" the Spice Girls and is "totally obsessed" with TV's Buffy the Vampire Slayer (note to the uninitiated: Buffy is cool. She wears pink miniskirts and knee-high boots and drives stakes through the hearts of monsters). On her last spending spree Alexandra (or rather, her mom) paid $135 for stuff from Delia's catalogue: blue face shimmer, wide-leg jeans, black sneakers, and a long-sleeved T-shirt with a maroon racing stripe down the sleeve. When asked what makes her and her peers different from Generation X, she shrugs: "I think we're more preppy. They liked that grungy stuff. It was, like, cool for them not to take a shower."

Head to toe, Alexandra's wearing brands hot with the girl-power set: a T-shirt by 26 Red Sugar; jeans by L.E.I. (that stands for Life Energy Intelligence); thick-soled sneakers by Vans.

Never heard of these brands? That's the point. On any given day at the mall, you can spot Alexandra and her friends and her friends' friends wearing baby T's by Free People, plaid pants from Tag Rag, super-wide-leg jeans by Paris Blues, drawstring skirts by Greed Girl, tank tops by Juicy, carpenter pants by Bulldog, nubby miniskirts by Dollhouse, slip dresses by Monster Girl, lug-soled boots by Skechers.

Levi's? Forget it. As if any self-respecting 14-year-old would ever wear the brand her mom and dad grew up in. According to Nancy Friday, author of My Mother/My Self (and, more recently, The Power of Beauty), the more that moms try to dress like teenagers, the more teens clamor for new brands, new accessories, new haircuts. "It's hard for today's adolescent to proclaim, 'This is my time,' when her mother wears her clothes, and looks as good as she does," Friday remarks.

That's why girl brands--like Jane by Sassaby--are hot. About four years ago, Howard Katkov (now 47) was struck by how advertisements for makeup and creams were all about solving problems: eliminating wrinkles, hiding dark circles, looking fresh. Teenagers don't have wrinkles; their mothers do. So in 1994, Katkov and his partners introduced Jane, a mass-market line of cosmetics in wonderful wild colors with seriously low prices and great names: Blue Funk, Shrinking Violet, Traffic Jam. The tag line: "If makeup isn't fun, why bother?" They didn't ask Cindy Crawford to pose for their ads; she's too old. Instead, they hired unknown 17-year-olds.

"When we first met with retailers, they said they didn't want teens in their stores because teens take things," says Don Pettit, 42, one of Jane's founding partners. In other words, they steal. "Girls weren't seen as a consumer, but as a problem." Less than four years after Jane was launched, its sales have reached an estimated $30 million. Last month Jane was bought by Estee Lauder Cos.

Dineh Mohajer, 25, got the idea for her company because she was bored silly by the range of nail polish on the market. All you could buy were colors for old fogies, colors like Serene Red and Love That Red. So in 1995 she started selling pastel blue and lime green nail varnish with names like Weenie, Dork, Porno, and Pimp. Last September she introduced a line of mascara and eye shadow: They're called Hyper, Girlie, and Spazz. How did she come up with these names? "I just said, 'You know what? We should name them really cute, fun names,' " she explains. Easy. Her company, Beverly Hills-based Hard Candy, now has sales of more than $10 million, devoted girl fans, and dozens of imitators.

In the world of girl power, big brands are an endangered species. Marketers sense this and suspect there may be a way around it: Pretend you still have to be discovered. Pretend the girls are in charge. "If the brand is No. 2 or 3 or 4--cheer it. If it's No. 1, play it down. That is what will get the brand accepted." That's thoughtful advice from Street Trends, a new book by the co-founders of Sputnik, a New York-based market research firm that specializes in youth culture.

But you don't need a high-paid consultant to tell you what makes the teenage girl tick. Today's power girl has more in common with Sinatra's bobbysoxers than she's letting on. The covers of Seventeen magazine don't still scream "10 Ways to Get Over a Crush" for nothing.

"Teens still have pimples, they still want to go to the prom, they still want to be popular," sighs Michelyn Camen, image director for Steve Madden Ltd. Or, in the words of 12-year-old power girl Jennifer Esner, she with the closet full of Steve Madden shoes: "When I grow up I want to be a pediatrician or a model."