Meta-analytic reviews have shown that the effects of grouping programs depend on their features. Programs that entail only minor adjustment of course content for ability groups usually have little or no effect on student achievement. In some grouping programs, for example, school administrators group students by test scores and school records and then expect all groups to follow the same basic curriculum. Under this approach, pupils in middle and lower programs learn the same amount as equivalent students do in mixed classes, while students in the top classes outperform equivalent pupils from mixed classes by about 1 month on a grade-equivalent scale. Self-esteem of lower aptitude students rises slightly and self-esteem of higher aptitude students drops slightly. Grouping programs that entail more substantial adjustment of curriculum to ability have clear positive effects on children. In cross-grade and within-class programs that provide both grouping and curricular adjustment, pupils outperform equivalent control students from mixed-ability classes by 2 to 3 months on a grade-equivalent scale. Programs of enrichment and acceleration, which usually involve the greatest amount of curricular adjustment, have the largest effects on student learning, with talented students from accelerated classes outperforming nonaccelerates of the same age and intelligence quotient by almost 1 full year on achievement tests. Talented students from enriched classes outperform initially equivalent students from conventional classes by 4 to 5 months on grade equivalent scales. (Contains over 200 references.) (Author/JDD)