Fortune’s 40 under 40 — The hottest young stars in business

2. Jack Dorsey

It was Dorsey who shared the world's first tweet, "just setting up my twttr," and conceived the idea to share a brief "status." Now, as Twitter readies its monster IPO, Dorsey is the only cofounder of the initial trio (Biz Stone, Ev Williams, and him) who still works at the company. Dorsey had moved on to found Square, the vaunted mobile-payment system that now processes $15 billion annually (where he's CEO), but in 2011 he added the role of Twitter executive chairman and still helps drive big decisions. (See Jack Dorsey: The pride of St. Louis.)

3. Mark Zuckerberg

It took 14 months, but shares of the social network finally topped their initial public offering price of $38 late this summer (now they're even higher). That's because Zuckerberg's mobile strategy at last appears to be working. Mobile-ad sales now account for 41% of Facebook's total ad revenue, and the number of people posting photos and status updates on the go surged 51% in the most recent quarter. Facebook is also reportedly planning to launch TV-style ads that will cost brands up to $2.5 million a day. In other words, investors' confidence in Facebook—and Zuck—is back.

4. John Elkann

Elkann was picked for the Fiat board at age 21 by his grandfather Gianni Agnelli, the dapper industrialist who built the company into a global powerhouse. After Agnelli's passing, Elkann, thrust to the forefront by the untimely deaths of two potential successors, was named chairman of Fiat and chairman and CEO of Exor, the family's $142 billion holding company with interests in commercial real estate, banking, media, heavy equipment, and the soccer team Juventus. Elkann—known as Jaki within the family—has led a striking turnaround: He stabilized Fiat and oversaw the successful investment in Chrysler, and has reorganized the far-flung family empire.

5. Rob Goldstein

The math whiz, born in Canarsie, Brooklyn, heads BlackRock Solutions, the asset-management firm's $518 million division that offers advisory services, risk management, analytics, and tech platforms to governments and institutional investors. He's added 150 clients to the firm's institutional business, and he's pioneering a new service-oriented approach to advisory while retooling a plan for a platform to circumvent bond brokers and their fees.

6. Brian Chesky, Joe Gebbia & Nate Blecharczyk

In the five years since they launched it from a San Francisco loft, Airbnb has single-handedly legitimized couch-surfing as a hotel alternative—and provided a not-insignificant revenue stream for entrepreneurial homeowners. It's also become one of the most watched companies in the tech world, valued at $2.5 billion. Now Chesky is CEO, Gebbia chief product officer, and Blecharczyk its CTO. Although Airbnb's reach already spans more than 33,000 cities and 192 countries, its biggest asset could be a growing trove of travel data about users.

7. Chris Weideman

This Stanford Law grad was tapped in April to become newly installed Treasury Secretary Jack Lew's right-hand man, just in time to help navigate the department rattling IRS scandal. Weideman earned the job by distinguishing himself as a go-to fixer for Tim Geithner, so he knows how to handle an emergency. Now he's sweating longer-term challenges too: a comprehensive rewrite of the tax code, the ongoing budget battles with congressional Republicans, and international headaches from Europe to China. That's a tall order.

8. Jun Wang

Imagine a grain of rice that can suck pollutants out of the soil; or a cancer treatment so targeted and personalized it exists for only one person—you; or a worldwide database of not just every human's genetic code but the genomes of every living thing, a sort of biological Google. This is the work of Wang, a genomics professor and the director of gene-sequencing center BGI (formerly Beijing Genomics Institute). The institute generates about six terabytes of data a day, and it often falls on Wang to see the forest through the trees. He is a busy man: scientist, professor, author of more than 100 papers. His ultimate goal, he says, is to take these extraordinary things and sell them to ordinary people. It's already working. That depollutant rice may soon come to market.

9. Tracy Britt Cool

Ever since Warren Buffett hired Cool, the daughter of Kansas farmers, out of Harvard Business School in 2009, he has heaped responsibilities on her. He named her chairman of four Berkshire-owned companies— Benjamin Moore, Johns Manville, picture-framer Larson-Juhl, and Oriental Trading—and recently put her on the board of H.J. Heinz. "She's a blotter for learning," says Buffett.

10. Liv Garfield

As strategy director for BT (formerly British Telecom), Garfield made the $4 billion case for building out fiber broadband to 19 million U.K. homes. Now, as CEO of BT's infrastructure division, she's doing it, managing more than 20,000 engineers. Recently named to the board of British megachain Tesco, she's said to be a contender one day for BT's top spot.

11. Ben Silbermann (TIE)

Despite all the hype, Silbermann has taken his time making money on the digital-scrapbook platform. But in the past year he's rolled out an analytics service and branded pages for businesses, perhaps greasing the wheels for monetization. Growth has continued: Pinterest traffic nearly tripled, to 53 million unique visitors this year.

11. Kevin Systrom (TIE)

Co-founded by photo fiend Systrom in 2010 and sold to Facebook for $1 billion in 2012, the mobile photo platform is thriving under its new parent, growing from 40 million monthly users to 150 million (with 50% outside the U.S.). The company has new video capabilities, a dedicated staff of 50—and a plan to incorporate ads to make money.

12. Chuck Myers

Myers is a bright star at Fidelity, where he manages $10.5 billion in small-cap stocks. You've probably never heard of his top holdings—companies like Berry Petroleum and j2 Global. But his picks during the financial crisis have driven his two mutual funds to outperform all but a handful of competitors over the past five years. His secret? Avoiding distractions. He doesn't have voicemail and spends most days inside his quiet office studying dozens of companies. Advice he would have given his 20-year-old self: Buy Apple.

13. Travis Kalanick

City dwellers don't have to flail their arms to hail a taxi thanks to Kalanick and co-founder Garrett Camp. Their smartphone app Uber lets users summon a black car or cab with a tap on a virtual map and track the driver's ETA. Uber is already available in 42 cities, from New York to Taipei, and isn't slowing down, despite noise from regulators: A new round of $258 million in funding values it at $3.4 billion (see Uber feature story).

14. Aaron Levie

Ever since he started Box in his dorm room in 2005, Levie has been running around (in bright red sneakers) trying to convince the world that enterprise software is cool. It appears his evangelistic approach is working. Box's file-sharing service for companies has amassed over 180,000 corporate customers, including P&G and Toyota. Sales are on track to more than double this year, and Levie has opened offices in Paris, London, and Munich to help boost sales overseas. The company has raised close to $300 million in funding, and many say it may go public sometime next year. Fun fact: In one summer Levie was rejected by Google, Goldman Sachs, Lehman Brothers (phew!), and Napster (phew!) for internships.

15. Ryan McInerney

By the ripe old age of 34, McInerney had risen to head J.P. Morgan Chase's consumer-banking division, overseeing 76,000 employees and becoming the youngest member of the megabank's executive committee. Earlier this year he left to become president of Visa. Expect him to shake things up: When he took over consumer banking at J.P. Morgan, its Chase branches trailed in consumer satisfaction. His fix: Gather all district managers in one room and play customers' complaints over a loudspeaker for an hour and a half. By the time he left, Chase had climbed in the rankings. "It's all about listening to the front line," says McInerney.

16. Jeremy Stoppelman

Part online business directory, part customer reviews hub, Yelp has become indispensable for 108 million active monthly users since ex-PayPalers Stoppelman and Russel Simmons started it in 2004. This year it took the concept further with Yelp Platform, letting users buy meals from restaurants directly on Yelp (Stoppelman says services will follow). But plain old advertising is expected to power revenue for the company in the years to come: Sales jumped 69% to $55 million last quarter, trumping Wall Street's expectations (see the feature story on Yelp).

17. Ross Martin

This cerebral, sneaker-collecting poetry major became Viacom's brand innovator for the MTV generation. After nearly a decade in film, Martin was recruited by Viacom as head of programming for MTVU. There he founded what is now called Scratch, an in-house marketing laboratory that partners with megabrands to come up with fresh ways to reach millennials. "Young people are turning these companies into what they want them to be," he says.

18. Catherine Lacavera

Google's secret weapon in the smartphone wars is attorney Lacavera, who joined the company in 2005 and has built Google's intellectual-property litigation team and established its aggressive defense strategy. She oversaw Google's 2012 victory over Oracle's IP attack on Android (Oracle has appealed); twice her team has triumphed against Viacom in long-running copyright litigation; and she also advised on Google's $12.5 billion Motorola Mobility acquisition in 2012. In June she negotiated a $490 million multiparty settlement with TiVo.

19. Ashish Thakkar

He survived the Rwandan genocide, dropped out of school, and lived out of a suitcase selling computers in Uganda at age 15. Since then, Thakkar has built the continent's largest pan-African conglomerate— operations span from manufacturing to agriculture to real estate, including Africa's biggest IT firm—with a portfolio of some $1 billion and 7,500 people across 21 countries. He also serves on the advisory panels to the Presidents of Uganda and Tanzania and runs a foundation that mentors young African entrepreneurs. Next up: Mara Messenger, Africa's first instant-messaging platform.

20. Will Adams

Yes, the guy you know as will.i.am is a bombastic pop star and the frontman of the Black Eyed Peas, but Adams has established himself as a go-to social and technology consultant big brands trust. He's advised Coca-Cola and Intel. He's teamed up with NASA, the NBA, Anheuser-Busch InBev, and inventor Dean Kamen. He just enrolled at MIT for computer science, proving he's more than another deep-pocketed enthusiast throwing money at other people's ideas. His line of technology accessories, i.am.plus, started with an iPhone camera lens and in 2014 will put out a home robot and a smartwatch. (Sure, the camera wasn't a hit, but it showed he has the ability to create.) Auto-tuned electro-warbler though he is, Adams may prove to be the new model for how pop stars attain business influence.

21. Avani Davda

The CEO of the joint venture between Starbucks and Tata Group, Davda is bringing iced Frappuccinos to India—a critical region for the coffee giant, which wants the country to become one of its top five global markets. Davda is not only the youngest but also the only female to head a business at the $100 billion Tata conglomerate.

22. Sam Yagan

At Harvard, Yagan and friends created the popular study guide SparkNotes and sold it for $30 million. Then they made a file-sharing service and scaled it fast before shutting down to avoid going to court. In 2003 he and the same friends thought they could make a free alternative to dating sites eHarmony and Match.com, so they created OkCupid, which pioneered the use of data mining in online dating. They sold it to IAC's Match.com in 2011 for $90 million, and last fall Yagan was made CEO of Match Inc. Other companies now use OkCupid's vast trove of social data, and Yagan, a self-professed introvert who is married to his high school sweetheart, stands as the foremost authority in a fast-growing $2 billion industry.

23. Lyndon Rive (TIE)

Rive founded his solar energy company with his brother in 2006 (their cousin is Tesla co-founder Elon Musk), and where many firms have stumbled, Rive's has taken off. Shares are up more than 300% since its IPO, and its 68,000 customers represent a sizable chunk of the 300,000 total residential installations in the U.S.

23. Cyrus Massoumi (TIE)

Six years after a scramble to see a doctor gave him the idea, more than 2.5 million people use Massoumi's medical appointment booking service. The company now has more than 400 employees, and has doubled the number of engineers in the past year.

24. David Karp

When Karp dropped out of high school at age 15, it wasn't without a life plan: "I wanted to drive software, and I wanted to build stuff on the Internet," he recalls. The rest is history: He built Tumblr, which hosts over 136 million blogs, and sold it to Yahoo earlier this year for $1.1 billion. Under its new parent—and CEO Marissa Mayer (see No. 1)—Tumblr is reportedly aiming for $100 million in sales this year from advertising, up from an estimated $13 million generated last year.

25. Ben Hulburt

In 2001, with an investment of $1 million, Army vet Hulburt co-founded Rex Energy, an independent energy outfit he grew into a $300 million company by buying up oil and gas fields before taking it public in 2007. With his brother, he then founded Eclipse, based in State College, Pa., a developer of natural-gas properties in the Appalachian Basin. The firm has 90 employees and an estimated $1 billion valuation—an IPO looks imminent. Downtime: Hulburt has a man cave in his home adorned with memorabilia honoring his two favorite football teams: the Buffalo Bills and the Penn State Nittany Lions.

26. Jonah Peretti

If online media had a Henry Ford, it would be Peretti, who for more than a decade has been studying—and ruthlessly optimizing for—the ways people get information on the Internet. His success at goosing the rankings of HuffPost stories in search results fueled the site's frantic growth; by the time he launched BuzzFeed in 2008, social networks were in bloom, hence the site's wellknown reliance on shareable listicles like "15 Tiny Hats on Cats." The formula is working: In August, BuzzFeed drew a record 85 million unique visitors—and a growing roster of heavyweight journalists help it break real news too.

27. Jeremy Bird

Bird rose from humble beginnings—he was raised in a trailer park outside St. Louis—to become national field director of Barack Obama's 2012 campaign. He merged technical wizardry with on-theground organizing to secure wins in eight of nine battlegrounds (and a second term for the President). Bird's gone mercenary with the new firm, offering the Obama campaign's magic to corporate interests, nonprofits, and political campaigns. One ambitious project: Battleground Texas, an effort to tilt the Lone Star State blue.

28. Daniel Ek (TIE)

Spotify has more than doubled its revenue for each of the past three years, but it has had to pay out $500 million in royalties since 2008 and will owe $500 million more this year. Still, users love it: 14 million new ones since the beginning of 2012.

28. Max Levchin (TIE)

The tech world knows this list returnee as a founder of PayPal and Slide, but now Levchin has launched Glow, one of the Valley's first fertility apps. Levchin is also keeping busy on the boards of Yelp and Yahoo.

29. Lani Hay

This Naval Academy grad has grown the threat analytics firm she founded in 2003—it does things like predict the location of IEDs—into a fast-growing government contractor with revenue of close to $40 million. Hay spotted opportunity: As a woman, a minority (her mother is from Vietnam), and a service-disabled veteran (her hearing was damaged while in the Navy), she knew she'd have an edge in the contracting process—and that the government has to expedite pay to small businesses. Hay has political ambitions: Her fundraisers and events are celebrity- and politico-studded.

30. Claudia Sender

Sender, a relative newbie to the airline industry, having joined TAM Airlines in 2011, made headlines earlier this year when she was given control of Brazil's largest airline (TAM Group revenues: $7 billion). The board is banking on the Harvard MBA grad, ex-Bain consultant, and Whirlpool marketer to rally the troops in an industrywide slump and fulfill the promise of the recent merger with Chile's LAN. The only woman in the merged carrier's C-suite, she pilots the biggest division, with almost 30,000 employees and Brazil's fifth-largest payroll.

31. Katia Beauchamp & Hayley Barna

Founded in 2010 by Harvard Business School grads Barna and Beauchamp, Birchbox providers its female users with four to five beauty samples for $10 a month. The startup popularized the subscription box trend, but don't categorize it as such. The monthly packages enable bite-size discoveries, leading more than half of all subscribers to purchase full size products on their website. Birchbox has raised $11.9 million to date and launched its men's line (with a higher price point of $20 per month) in April 2012. It then acquired Parisian JolieBox the following September, allowing easy expansion into France, Spain and the UK, growing its user-base upwards of 400,000.

32. Niraj Shah

You may not have heard of Wayfair, but neither have a lot of the people who've turned the online purveyor of home goods (faux florals, ceiling fans, fire pits) into a nearly $1-billion-in-sales business. CEO Shah and his Cornell classmate Steve Conine started out with racksandstands.com in 2002 and quickly expanded, buying dozens of niche domain names and launching many from scratch—simplydogbeds.com, justshagrugs.com, dinnerplates.com—and in 2011 changing the company name from CSN Stores to Wayfair (700 employees went on a Boston bar crawl in Wayfair T-shirts to help market the new name). Today under three main sites—Wayfair, flash-sales site Joss & Main, and the higher-end AllModern.com—it's the biggest online-only homegoods seller, and this summer it acquired New York retailer DwellStudio. The Boston business community is keeping close watch for an imminent IPO. We bet Jeff Bezos is too.

33. Rachel Shechtman (TIE)

This TEDx-talking retail guru who has consulted for brands like Kraft, TOMS shoes and Lincoln has earned a reputation for thinking outside the box when it comes to the future of shopping. Step into her two-year old (and profitable) New York boutique to see retail reconceived: one night an Italian grandmother gives pasta-making lessons; on another you'll find installations sponsored by the likes of General Electric, Pepsi and HP. Something you won't see? The same old stuff: Shechtman changes the theme, merchandise and "story" every month or two. Retail biggies like Nordstrom and CMOs of Fortune 500 companies have been checking it out to get her read on where retail is headed. Fun fact: loves to cold call.

33. Alli Webb (TIE)

Drybar founder Alli Webb turned blowouts into a booming business. Since 2010, Webb, who runs the company with her brother, Michael Landau and her husband, Cameron Webb, has opened 30 blow dry salons (no cuts, no color) across the country, serving up a streamlined menu of styles to some 100,000 customers a month. They're on track to hit $40 million in revenue this year, thanks also to a new line of styling products and tools, which beauty retailer Sephora picked up in an exclusive deal.

34. Danae Ringelmann, Slava Rubin & Eric Schell

The trio behind the crowdfunding site bonded when they learned they each had faced trouble raising money in different industries: Ringelmann for indie films, Rubin for a charity, and Schell for a theater company. They launched Indiegogo in 2008 (ahem, a year before Kickstarter); users like that it's global and allows all projects—and that there's an option to keep money raised even if they don't hit their goal. That may be why 90% more campaigns were launched in the past year.

35. Kristin Groos Richmond & Kirsten Saenz Tobey

Richmond and Tobey started in 2006 with a small pilot program in Oakland, serving healthy lunches in five schools. Today their RevFoods delivers 200,000 meals a day at 1,000 schools. Richmond, CEO, and Tobey, chief impact officer, who met at UC Berkeley's Haas School of Business, recently expanded into grocery stores with a rival product to Kraft's Lunchables. With just under $70 million in revenue, the company is on a path to profitability and has backing from the likes of Catamount Ventures and Oak Investment Partners.

36. Rachel Haot

As the country's first chief digital officer in government, Haot was Mayor Michael Bloomberg's pick, tasked in 2011 with making New York City a tech-friendlier hub. Her team launched the WeAreMadeinNY.com campaign, which maps some 3,000 job openings at more than 1,000 Silicon Alley companies. The NYC.gov site is getting a face-lift thanks to a city-sponsored hackathon, and the city government's social media traffic has nearly tripled.

37. Dhiraj Rajaram

This former consultant (Booz Allen and Price Waterhouse) has rocked the world of big data with his eight-year-old firm, which provides data and analytics services to a quarter of the companies on the Fortune 500. Recent investments from MasterCard and Fidelity have pushed the Sequoia- and General Atlantic-backed company's valuation to more than $1 billion and made it a rumored buyout target. Analysts at the 2,500-person firm, based in Chicago, Bangalore, and Austin, are trained at an in-house math factory called the Mu Sigma University; Rajaram, who grew up in Chennai, runs it from India and the U.S. The company's name comes from the statistical symbols for "mean" and "standard deviation," and conference rooms are named after statistical distributions.

38. Andrew Ng

Massive open online courses (MOOCs) are all the rage these days, and Stanford computer scientist Ng's Coursera is the world's leading provider of them. More than 4.5 million people have signed up for Coursera's 400-plus MOOCs, and the company has raised $43 million this year—bringing its funding to $65 million from backers including Kleiner Perkins Caufield & Byers and the World Bank.

39. Tamara Ralph & Michael Russo

In six years the bespoke clothing label founded by these two Aussies has attracted a clientele that includes Angelina Jolie and royals the world over. R&R's handmade pieces fetch up to $500,000 and range from baby garments to Beyoncé's crystal-studded stage wear. Nimbler and more exclusive than the big houses, R&R gets gowns to the rich and famous faster (four weeks instead of six months) and with a white-glove touch (Cartier on site; chauffeured rides in a Rolls-Royce). Ralph, a fourth-generation couturier who sold dresses to Sydney's socialites as a teen, handles the fashion; Russo, the finances—the company is growing 400% a year.

40. Kat Cole

Three years into her stint at the helm, Cole is transforming private equity-owned Cinnabon into a multichannel consumer brand. Taco Bell and Burger King sell its licensed products, its cinnamon is now in some 60 items on grocery-store shelves (think Pillsbury cinnamon rolls), and sales of all Cinnabon-related products are approaching $1 billion. An engineering major who worked at Hooters to pay for college at the University of North Florida, Cole wanted to become an attorney for DuPont—but after Hooters sent her around the world to help open new markets, she dropped out and worked her way up to a vice president at the chain.