Age: 48
Occupation: CEO, Twitter
Twitter CEO Costolo has one of the toughest jobs in tech: turning the wildly disruptive communications platform into a money-making business while not alienating its millions of users who have grown accustomed to the lack of intrusive ads. A former improv comedian, Costolo sent the following as his first tweet at the company: "First full day as Twitter COO tomorrow. Task #1: undermine CEO, consolidate power." One year later, he was the boss.