Peer relations is one of the most active research fields in educational psychology and developmental psychology in Japan. The present article provides a brief-review of Japanese peer-relations studies in the last decade from 4 different perspectives:(1) formation and development of peer relations and friendships: formation of peer relations, social status, friendships during adolescence, and group culture,(2) social behavior and peer relations: internalizing problem behavior, externalizing problem behavior, and social skills,(3) cognitive aspects of peer relations: intent attribution, generation and estimation of response strategies, and goal setting and strategy choice, and (4) emotion regulation and peer relations. The research agenda of Japanese peer-relation studies is also discussed.