The research on the effects of class size is summarized in this review. It includes (1) an introductory examination of class size definitions, statistics, and research design; (2) a summary of the research on class size, including the effects of class size on student achievement for grades K-8 and 9-12 and on the classroom environment; (3) teacher and public opinion on class size; (4) examples of the budgetary impact of altering class size; and (5) some of the policy implications of class size. The author advises school decision-makers to go beyond the customary generalizations about class size and ask instead, "Which types of students might benefit the most from smaller classes?" The appendix contains statistics on pupil-teacher ratios for states and large cities. (Author)