The purpose of this paper is to summarize the issues and principal findings of research on homogeneous and heterogeneous ability grouping, and to consider the implications it may have for evaluating and improving the design of educational settings. To begin with, definitions of terms used in the discussion are given. The paper then proceeds to sketch research findings on academic achievement, affective development, and ethnic and socioeconomic consequences, and concludes by discussing the implications of research in these areas. (JW)