Using a sample from the NICHD Study of Early Child Care (N = 435; 219 girls), the authors derived several measures of regulation and dysregulation that predicted, both concurrently and longitudinally, children's positive and negative peer interactions in multiple contexts. Observers rated peer interactions in child care and during dyadic play with a friend, and mothers rated peer behavior. The authors based the derived measures on resistance to temptation (36 months) and delay of gratification (54 months) tasks, as well as observations in child care of children's compliance and defiance with adults at both ages and maternal reports. Preschoolers who had better impulse control and who were more compliant and less defiant with adults engaged more often in friendly, positive, peer play and were less negative in their peer play across contexts. Associations between regulation and dysregulation and peer interaction were broader and more consistent at 54 months than at 36 months. Longitudinally, regulation at 36 months was only modestly associated with more positive and less negative peer play at 54 months. The authors discuss findings in the context of developing self-regulation and its importance for early peer relationships. (Contains 10 tables and 2 notes.)