40 under 40

Our 2013 edition of the list features young hotshots who are rocking businesses as diverse as air travel, natural gas, and genetic coding, as well as—if you can believe it— government.

23. Cyrus Massoumi (TIE)

40u40 cyrus massoumi
  • Age: 37
  • Title: Founder and CEO
  • Company: ZocDoc

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.

  @FortuneMagazine - Last updated September 30 2013 03:41 PM ET
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