The knowledge base regarding use of instructional grouping in the classroom includes findings from research on effective schools, effective teaching, student academic achievement, student perceptions of self and others, student motivation, student attitudes toward school, and student friendships and interactions in the classroom and school. A dominant theme in the research findings is that some types of instructional grouping contribute to more positive academic and affective outcomes for students. Other groupings, particularly stable, long-term groups based on student ability, have an adverse effect on students. This paper synthesizes this research, defining instructional grouping and describing the following approaches: learning cycle groups, cooperative groups, and long-term ability groups. Remaining sections focus on why instructional grouping is used, actions for effectiveness, cautions regarding use of instructional groups, and policy implications with respect to use of instructional groups and teacher training. Thirty-three annotated references are included. (TE)