Using a best evidence synthesis model, this book reviews research conducted in primary/elementary and secondary schools in the United States and in Great Britain on setting and streaming (ways of grouping pupils according to ability). The review concludes that research provides no evidence that student achievement at the primary/elementary school level is raised either by streaming (grouping by general ability) or setting (grouping by subject ability) within the school. Within-class ability grouping at the elementary school level has been found to have advantages for mathematics achievement. At the secondary level, there is no consistent and reliable evidence of positive effects of setting and streaming in any subjects, or for students of particular ability levels. Evidence suggests that teachers sometimes treat mixed-ability groups as though they were low-ability groups and that even experienced teachers frequently use inappropriate whole-class teaching methods with mixed-ability classes. What is important in determining achievement is that the challenge and support given to learners meet their needs. (Contains 168 references.) (KB)