Class size is one of the small number of variables in American K-12 education that are both thought to influence student learning and are subject to legislative action. Legislative mandates on maximum class size have been very popular at the state level. In recent decades, at least 24 states have mandated or incentivized class-size reduction (CSR). The current fiscal environment has forced states and districts to rethink their CSR policies given the high cost of maintaining small classes. The substantial expenditures required to sustain smaller classes are justified by the belief that smaller classes increase student learning. The authors examine "what the research says" about whether class-size reduction has a positive impact on student learning and, if it does, by how much, for whom, and under what circumstances. Despite there being a large literature on class-size effects on academic achievement, only a few studies are of high enough quality and sufficiently relevant to be given credence as a basis for legislative action. (Contains 31 footnotes.)