IN THE 1980s activists were called corporate raiders and were the jackals of capitalism. Today activism is mainstream and arguably the biggest preoccupation of American boardrooms. Since 2011 activists have helped depose the CEOs of Procter & Gamble and Microsoft; 15% of the members of the S&P 500 have come under attack since 2009, among them the world's most valuable company, Apple. The Economist estimates that the about half the S&P 500 have had an activist on their share register over that same period. One of Silicon Valley's biggest firms admits, "We think about an attack all the time." Of the 8,000 known hedge funds only around 71 are considered to be activist funds, less than 1% of the total. But with 4% of the total assets under management they punch above their weight. Our table above shows the biggest funds, which are differentiated by their vintage, staying power and propensity to campaign—and by their belligerence once the game is afoot.