This paper synthesizes what is known from previous research about the effects of children teaching other children in different K-12 contexts, emphasizing the academic and affective impact of such activity on the child doing the teaching. The study reviews peer teaching, integrating research findings about different types of peer teaching into one analysis. It assesses the extent to which the development of teaching abilities is good general education for students and identifies areas related to the topic that are insufficiently researched. Data are presented on (1) the effectiveness of standard peer teaching approaches: cross-age and same-age peer tutoring, cooperative learning, reciprocal peer tutoring, and reciprocal teaching; (2) conditions affecting peer teaching outcomes: contextual features (e.g., group rewards, individual accountability, and the composition of the cooperative learning group or peer tutoring dyad); student characteristics and behaviors (e.g., children who provide detailed and complex explanations to other children learn the most in peer tutoring and cooperative learning contexts); and (3) what it is about teaching others that leads to learning (e.g., the cognitive effects of explaining to others and arguing with others). (Contains 56 references.) (SM)