Seven- to 9-year-old boys (N = 177) and their mothers participated in this study in which the associations between boys' experiences with their mothers, their beliefs about familiar and unfamiliar peers, and their peer adjustment were examined across a 2-year period. Boys' negative behavior with mothers was associated with their having more negative beliefs about familiar and unfamiliar peers and with their being more aggressive and less well-liked. Beliefs about familiar peers predicted changes in boys' social acceptance, whereas negative beliefs about unfamiliar peers predicted changes in aggression. In addition, boys' beliefs about peers changed in response to their social experience. The implications of these findings for children's social development are discussed.