The seven brief chapters of this paper advocate mixed-age grouping in schools and child care centers. Discussion defines mixed-age grouping and examines some limitations of single-age grouping. Research findings on social and cognitive aspects of mixed-age grouping are reviewed. Social aspects are discussed by considering in turn the following statements: older children exhibit facilitative leadership, mixed-age grouping promotes prosocial behaviors, children appear to play as freely in mixed-age groups as in same-age groups, self-regulation appears to improve, social participation is heightened for younger children, older children create complex play for younger ones, older children operate well in younger children's zone of proximal development, and younger children allow isolated older children social skills practice. The therapeutic effects of mixed age interaction are also considered. Cognitive aspects are discussed in terms of effective cognitive conflict resulting from peer interaction, complex aspects of cognitive conflict, novices and experts in mixed-age groups, and children's sensitive adjustment of communication for listeners. Additionally, successful multi-age programs and some effective teaching strategies are described, along with peer tutoring and cooperative learning. Concluding materials present recommendations for decision makers in schools and centers for young children. Suggestions for teachers implementing mixed-age grouping are appended. Over 60 references are cited. (RH)