When students enter middle school, they face 2 major challenges, one involving the biological, cognitive, and socioemotional changes they are going through, and another involving the transition from elementary to middle school. Peer learning has considerable influence on how well they manage these challenges. The research that exists on peer learning, however, indicates it can have positive or negative effects on students' achievement and well-being, depending on the nature of the program and how it is implemented. Perhaps the most important peer learning program is cooperative learning, because it is based on social interdependence theory and the hundreds of research studies. The research indicates that cooperation promotes greater efforts to achieve, more positive relationships, and greater psychological health than do competitive or individualistic efforts. In middle school students the quality of peer relationships accounts for 33 to 40% of the variance in achievement of middle school students. These outcomes are expected only when cooperative learning is structured to include positive interdependence, individual accountability, promotive interaction, appropriate use of social skills, and group processing. Based on these results, 3 types of cooperative learning have been operationalized: formal cooperative learning, informal cooperative learning, and cooperative base groups. In implementing the 3 types of cooperative learning, teachers will want to encourage the development of personal relationships, which in turn will tend to increase their academic and social integration into middle school.