This literature review focuses on the effects of class size on styles of teaching practice, and on pupil behavior and attainment. Most of the literature is from the United States. Evidence from previous reviews, correlational studies, meta-analyses, and experimental interventions are presented in this document. Much of the existing evidence is at best confusing, sometimes even contradictory. However, there is sufficient evidence, mainly from American studies, to show that class-size reduction is associated with improvement in pupil achievement, with major benefits accruing in class sizes of 20 pupils or less per teacher. These benefits are more marked for grades K-3. Teachers believe that class size affects their teaching practices, in particular how they organize within-class groups and devote time to individual children. There is a paucity of evidence on the effects of class size on pupils' learning. There is little evidence of collaborative learning among pupils. Alternative approaches to organizing within-class and across-year groupings, more one-to-one tutoring from teachers and classroom assistants during the working day, and peer tutoring are alternatives that now need to be evaluated. An appendix presents the literature search strategy. (Contains 72 references.) (RT)