This paper argues that efficiency-oriented approaches to corporate governance and law are limited in their ability to explain the politics of corporate control and, in particular, the rise of shareholder activism. Politics, like other social action, is embedded in social structures that influence whether, when, and how collective action is accomplished by interest groups. We use a social movement framework to explain the changing capacities of shareholders and managers-as members of classes-to act on their interests in control at the firm, state, and federal level. We illustrate this framework by showing how activist shareholders increased their influence in corporate governance in the early 1990s.