Hedge fund activism is a new form of investment strategy. Using a large hand-collected dataset from 2001 to 2006, we find that activist hedge funds in the United States propose strategic, operational, and financial remedies and attain success or partial success in two-thirds of the cases. The abnormal stock return upon announcement of activism is approximately 7 percent, with no reversal during the subsequent year. Target firms experience increases in payout and operating performance and higher CEO turnover after activism. We find large positive abnormal return to hedge fund activists, which is higher than the return to other equity-oriented hedge funds.