This research review summarizes two major sets of meta-analyses on five kinds of ability grouping programs: (1) XYZ classes (high, middle, and low classes); (2) cross-grade grouping; (3) within-class grouping; (4) accelerated classes; and (5) enriched classes. One group of meta analyses concluded that the strongest benefits from grouping were found in programs in which there was a great deal of adjustment of curriculum for highly talented learners. The other meta-analysis did not find any strong positive benefits of grouping, but did not examine grouping programs designed for highly talented students. Re-analysis of all studies included in both sets of meta-analyses confirmed that higher aptitude students usually benefit academically from ability grouping. Benefits are in proportion to the amount of curriculum adjustment, with programs entailing acceleration of instruction resulting in the most gain on standardized tests. Grouping was found to have less influence on the academic achievement of middle and lower aptitude students. Analysis of noncognitive outcomes suggests that the effects of grouping on self-esteem measures measures for all ability groups are small and may even be rather positive. Results are contrasted with the conclusions of J. Oakes ("Keeping Track: How Schools Structure Inequality" (1985). The review concludes that American education would be harmed by the wholesale elimination of programs that group learners for instruction by ability. (DB)