Activist investing takes places in even the largest multinational firms, as was the case with Bill Ackman of Pershing Square Capital Management, who forced McDonald's (MCD - Get Report) to make vital changes to its menu, improve customer service and realize hidden real estate value, all of which ultimately led to a doubling in McDonald's stock price.

Another large multinational firm engaged with an activist investor is Yahoo! ( YHOO). Now-board-member Carl Icahn has so far been unsuccessful in his attempts to get Yahoo! to sell its search business -- or even the entire company -- to Microsoft ( MSFT - Get Report).

But sometimes the activist investor isn't just another hedge fund. Take GSI Group ( GSIG). Stephen W. Bershad, the CEO of Axsys Technologies ( AXYS), put his own capital on the line when he recently bought 3.3 million shares of GSI Group in the open market.

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