"The critics are mistaken. Class size matters. Class size is one of the most-studied education policies, and an extremely rigorous body of research demonstrates the importance of class size in positively influencing student achievement. This policy brief first reviews the research on class size. Special attention is given to the literatures in economics and related fields that use designs aimed at disentangling causation from correlation. It then documents the recent rise in class size and considers how to compare the effects of class-size reduction with other commonly discussed policy alternatives."