Intended for teachers and administrators, this document provides an annotated bibliography of research since 1960 that deals with effects of grouping elementary school students, below the junior high level, according to their ability. The 34 annotations are arranged in three categories: research concerned with the effects of ability grouping on students' (1) academic achievement, (2) affective development, and (3) academic achievement and affective development. Within each category, studies focusing on interclass ability grouping are presented first; studies dealing with intraclass grouping second. The majority of the studies concern interclass ability grouping. Since intraclass ability grouping, especially in reading, is so widespread in American elementary classrooms, representative reading studies are included. Findings are summarized in a five-page discussion. The research indicated that high-ability groups benefit and low-ability groups are hindered by interclass ability grouping. Most researchers concluded that interclass ability grouping is a questionable instructional practice. Researchers supported intraclass ability grouping, but identified potential negative effects on learning outcomes. Findings regarding effects of within-class ability grouping on students' affective development were inconclusive. (RH)