FORTUNE — Talk candidly with corporate directors and they’ll tell you their No. 1 worry is no longer the economy or the chance of an accounting scandal — it’s activist investors. That’s because activists have taken on a new role this year, proving they can influence the world’s largest, most famous corporations by buying tiny stakes and then telling the board exactly what they think it’s doing wrong.
The change in activists’ tactics and influence has been dramatic (see chart below). Just three years ago, the activists’ biggest targets were mostly smaller companies in which the activists took significant stakes in order to get the boards’ attention. Now no company is too big — Apple (AAPL), a target of Carl Icahn and David Einhorn’s Greenlight Capital, is the world’s most valuable company — and stakes smaller than 1% are often enough to get a hearing with the directors or top executives.
The main reason for activists’ new swagger is that they’re deploying far more capital than ever (see chart above). Look for the growth to continue so long as activist investing keeps working: Activist funds overall are beating the market, and major pension funds have grown more willing to hand over a chunk of their trillions.
This story is from the October 07, 2013 issue of Fortune.