Although some literacy researchers consider grouping students for reading instruction to be a proven educational practice, the support for this belief is lacking from a research synthesis perspective. With this idea in mind, Slavin comments in the middle of his review on the effects of grouping: "there is not enough research on within-class ability grouping in reading to permit any conclusions" (Slavin, 1987, p. 320). Because of this, the question of whether "and" how to group students is often cast and answered ideologically rather than empirically. This review attempts to see if the empirical research available can answer either or both of these questions. It will contribute to the literacy field by focusing specifically on reading outcomes for classroom teachers, who instruct a wide variety of students. Informed by previous research on within-class grouping, the following three research questions guide the present study: (1) To what extent does within-class grouping impact student achievement in reading?; (2) For which grade(s) or which students is within-class grouping most or least beneficial?; and (3) Do any moderators, especially those identified by previous research (measurement source, teacher development, and grouping type), help explain this effect? (Contains 3 figures.)