I confront three models of the policy impact of social movements with data on the mobilization of ecology, antinuclear, and peace movements in the United States between 1975 and 1995 by means of time-series analysis: the direct-effect model, the indirect-effect model, and the joint- effect model. My analysis suggests that social movements have little, if any, impact on public policy and that, if they are to have an impact, it depends on the combination of overt protest activities, the type of issues they raise, and external resources such as public opinion and political alliances with institutional actors. Thus, it appears that, if they are to have a policy impact, movements need the joint occurence of mobilization, support from political allies, and public opinion favorable to the cause.