Behind some of the most momentous changes in American politics and history lie the struggles of ordinary people. The actions of farmers and workers spurred the expansion of national economic regulation to protect against the predations of big business. The political mobilization of seniors during the Great Depression fostered the development of the welfare state. Civil rights activists won landmark legislation to ban racial employment discrimination and integrate American society. Feminist agitation achieved a host of legislative victories to challenge gender inequalities; and environmental activists brought greater oversight to bear on industries throughout the private sector. Few doubt that social movements matter; indeed, it is difficult to contemplate either the pattern of national political and economic development or changes to inegalitarian social practices without them. This investigation thus begins with a simple question: How were these victories achieved or, more precisely, why do social movement targets give in to demands? Although considerable research has been dedicated to the explanation of movement emergence and development, only more recently has attention been devoted to movement outcomes. Such studies highlight movement characteristics, including differences between violent and nonviolent tactics, while others concentrate on external circumstances and support from third parties. Yet this research has often produced contradictory or inconclusive findings. To resolve these puzzles, I suggest that it is useful to start with the most elementary propositions about movement outcomes and then, piece by piece, assemble a general theoretical explanation.