This paper provides an overview of research and theory on within-class grouping for instruction, including how groups are formed and managed, how students interact in groups, and how grouping affects students' achievement, with particular concern for low-achieving students served by Chapter 1 of the Education Consolidation and Improvements Act of 1981. After reviewing the traditional theoretical approaches to the study of grouping in classrooms, the paper introduces a model that serves as a framework for integrating research on grouping, followed by a review of the major findings and issues in this literature. Finally, the following implications of the research for educating Chapter 1 students are given: (1) When assigning instruction groups, teachers must especially consider individual students' needs and characteristics. (2) Teachers should not be reluctant to reassign students to groups or alter the groups. (3) Homogeneous ability grouping is detrimental to learning of students assigned to low groups. (4) Heterogeneous ability, all-student groups and some variants to cooperative learning may be effective for both low- and high-achieving students. (5) Teachers should be knowledgeable about the variety of grouping practices and be able to use them. (6) Students should learn how to interact effectively in small groups. And, (7) instruction and assistance should be appropriate to the students' level and skills. An extensive list of references is appended. (PS)