Relations between children's peer play competence and other relevant competencies were investigated using two samples of urban Head Start children. Dimensions of peer play were examined concurrently with emotion regulation, autonomy, and language. Children exhibiting high levels of peer play interaction were found to demonstrate more competent emotional-regulation, initiation, self-determination, and receptive vocabulary skills. Assessments of positive engagement in play early in the year were associated with lower levels of aggressive, shy, and withdrawn adjustment problems at the end of the year. Children who successfully interacted with peers early in the year evidenced greater cognitive, social, and movement/coordination outcomes. Disruptive and disconnected peer play behaviors were associated with negative emotional and behavioral outcomes. (Contains 5 tables.)