"Get in over your head" is Mayer's guideline. It's this career philosophy that has fueled her turnaround effort at Yahoo since she joined the company as CEO last year, at age 37, after more than a decade at Google. Mayer has replaced key Yahoo executives, made a string of acquisitions (22 so far, including Tumblr—see No. 24—for $1.1 billion), and overhauled Yahoo's culture and HR rules (its logo too) in an effort to make the company innovative again. Revenue growth is her challenge, but investors like what our newly crowned No. 1 has done: The stock is up 86% since she came in.