These are the 21 richest American entrepreneurs under 40 - and they all made their fortunes in tech

Net Worth: $1.05 billion

Age: 32

Evan Sharp is a cofounder of Pinterest, which he helped start as a side project with friends while at architecture school back in 2009. What started as a fun project has now boomed into a site with 47.66 million unique monthly visitors in the US, and which is now valued at $11 billion.

Brian Sheth showing off Vista Equity's snazzy fashion sense at #Buyouts Texas pic.twitter.com/ya4TIKxJte

— Luisa Beltran (@LuisaRBeltran) December 10, 2013

Net Worth: $1.1 billion

Age: 39

Sheth is cofounder and president of Vista Equity, the technology-focused private equity firm founded in 2000. He is actively involved in his company’s investments and sits on 16 separate boards of Vista Equity funded businesses. Sheth additionally serves as a director at Waterfall Mobile, MarVista Entertainment, and Global Wildlife Conservation.

Net Worth: $1.2 billion

Age: 32

Houston found an interest in computers at two years old before jumping into the startup scene in high school. Dropbox may be his current venture, which he cofounded at 24, but this young entrepreneur had already worked at several startups before running the now $10 billion company.

Net Worth: $1.5 billion

Age: 36

The cofounder and CEO of the $10 billion co-working space startup WeWork was born in Israel and now lives in New York. He told Fortune the best piece of advice he ever got came from his wife, Rebekah: “She taught me that, ‘Financial success can never be the goal, only a byproduct of living with purpose.’ That was a game changer for me.”

Net Worth: $1.5 billion

Age: 32

Uber is now valued at an insane $51 billion. But Graves started as a database administrator for GE Healthcare in 2008, a job he described as "unglamorous." So he decided to drop the job and worked for free as an intern at Foursquare. Then when he was looking for work in 2010, he replied to a Tweet looking for new hires from Uber CEO Travis Kalanick, and the rest is history.

Net Worth: $1.57 billion

Age: 33

Ben Silbermann is CEO and cofounder of Pinterest. Silbermann is helping guide Pinterest in a new direction, as he said during the 2015 Re/Code Code Conference: “We’re laser focused on this idea that Pinterest will be the place where you discover.” Silbermann used Pinterest to plan his wedding and find the photographer.

Net Worth: $1.8 billion

Age: 27

Bobby Murphy is cofounder and CTO at Snapchat, which got its start with the more awkward name "Picaboo." He and cofounder Evan Spiegel reportedly didn't actually get along that well when they first met at the Kappa Sigma fraternity house at Stanford in 2011. The larger personality of Spiegel reportedly didn’t mesh well with Murphy’s more zen-like state.

Net Worth: $2.1 billion

Age: 25

Evan Spiegel is Snapchat's cofounder and CEO. After launching the app with two frat brothers, Frank Reginald Brown and Murphy, from his dorm room, he decided to drop out of Stanford a few credits shy of a degree. It appears that he made the right move, as Snapchat is now valued at $19 billion.

Net Worth: $2.1 billion

Age: 37

Robert Pera heads Ubiquiti Networks as its chairman and CEO. He was a hardware engineer at Apple Computers before founding his company. Pera also owns the NBA team the Memphis Grizzlies.

Net Worth: $2.2 billion

Age: 38

The entrepreneur now runs the two tech giants: Square and Twitter. He’s been busy preparing for Square’s entry into the public markets and slimming down Twitter to become more efficient.

Net Worth: $2.5 billion

Age: 35

Silicon Valley bad boy Sean Parker was the cofounder of Napster and founding president of Facebook. He also serves on the board of Spotify, and has been helping to build Brigade, an app that aims to revamp civic engagement.

Net Worth: $3.3 billion

Age: 34

Joe Gebbia is Airbnb's chief product officer, and was the one that brought the company's two other cofounders together. Gebbia describes Airbnb's genesis as "three airbeds, a pop-tart breakfast, welcoming hosts and an insider’s guide to the city." Now it's a force that's transforming the way we think about travel.

Net Worth: $3.3 billion

Age: 34

Brian Chesky is the CEO of Airbnb, and met cofounder Joe Gebbia while they were students as RISD (The Rhode Island School of Design). Chesky studied industrial design. The friendship stuck and the pair later decided to found Airbnb after mutually quitting their jobs while living in San Francisco.

Net Worth: $3.3 billion

Age: 32

Airbnb's CTO and cofounder Nathan Blecharczyk started coding when he was 12. He was talked into joining the creation of Airbnb by his former roommate, Joe Gebbia. Before Airbnb, Blecharczyk was a product manager at Microsoft.

Net Worth: $4.5 billion

Age: 31

Elizabeth Holmes dropped out of Stanford when she was a sophomore to found the healthcare startup Theranos. Built on secretive medical technology, Holmes turned Theranos into a powerhouse company and became America's youngest woman billionaire — at least on paper. But recently, there has been controversy over how effective Theranos' proprietary technology actually is.

Net Worth: $6 billion

Age: 39

Travel Kalanick is the feisty CEO and cofounder of Uber. His company has fought rivals and regulators as it has transformed from a black-car service into a sprawling logistics company gunning for a future of self-driving cars.

Net Worth: $6.6 billion

Age: 37

Before cofounding Uber, Garrett Camp helped create the web discovery platform StumbleUpon. The Canadian entrepreneur seems to always have his hand in a variety of projects. He is also the founder of Expa, a startup studio that helps other founders launch their own products.

Net Worth: $8.8 billion

Age: 39

Jan Koum is the cofounder and CEO of WhatsApp, the hugely popular mobile messaging platform Facebook bought in 2014 for $19 billion. Koum is originally from Ukraine and moved with his family to the US when he was 16 years old. His family initially struggled in their new country, living on welfare and food stamps. Perhaps this is one reason Koum is said to be incredibly down-to-earth, especially about his newfound wealth.

Net Worth: $9.7 billion

Age: 31

Dustin Moskovitz was Facebook's third employee and first CTO. He went on to found software firm Asana in 2008. With his wife Cari, Moskovitz later started Good Ventures, a philanthropic foundation.

Net Worth: $47.1 billion

Age: 31

Facebook CEO Mark Zuckerberg has been a billionaire on paper since he was just 23 years old. His company continues to grow, and attracted more than a billion users daily this September. Facebook's stock hit all-time record highs in November, driving up his wealth even further.