This study tests the hypotheses that (i) shareholder activism is beneficial to shareholders of targeted firms and (ii) shareholder activism is beneficial to shareholders of potentially targeted firms (there are spillover effects of activism). These hypotheses are tested by analyzing the effects of shareholder activism on target firm governance structure, shareholder wealth, and operating performance. The primary sample consists of 51 firms that were subject to shareholder activism by an institutional investor (CalPERS) between 1987 and 1993. To test for spillover effects of activism, control samples of potential activism targets (including an industry-matched sample) and firms insulated from activism are used in tests of changes in performance.