Social science research suggests that reducing class size has its largest effects on the achievement of minority and inner-city children during the first year of formal schooling. Despite scholarly disagreements about the implications of specific studies on class size, economists generally agree that targeted class-size policies rest on stronger evidence than untargeted policies. In this paper, the author argues that the influence of social science research on class-size policy depends partly on political context. Instead of simply asking whether research affects policy, it is equally important to examine how the politics of the class-size debate shape the many uses of research in the policy arena. This paper examines the influence of research on class-size policy in three sections, organized chronologically. First, it reviews empirical research on the relationship between class size and student achievement. Second, it describes the role that research played in shaping class-size policies in Tennessee and Indiana in 1989 and in Wisconsin and California in 1996. Each pair of case studies illustrates substantial differences in the way state legislators framed the class-size debate and used research in determining the scope and size of their respective policies. Third, it discusses several lessons bearing on the relationship between research and policy as applied to class size in particular and education reform in general. (Contains 1 table and 73 notes.)