This ERIC Digest summarizes Robert E. Slavin's 1986 comprehensive review of research on the different types of ability grouping in elementary schools. Slavin examines evidence on the achievement effects of five comprehensive ability grouping plans in elementary schools: (1) ability grouped classroom assignment; (2) regrouping within grade level for reading and mathematics; (3) the Joplin plan--regrouping across grade levels for reading; (4) the nongraded plan--grouping according to performance rather than age; and (5) within-class ability grouping. The digest summarizes the effects found, and presents Slavin's recommendations for schools and teachers regarding ability grouping. (PCB)