Longitudinal links between early childhood temperament, maternal sensitivity, and adolescents' adjustment have been proposed and found in several longitudinal studies, but the mechanisms of influence have not been explored. The authors examined the paths from maternal sensitivity and temperament in early childhood to adolescents' prosocial, aggressive, and delinquent behaviors via childhood social behaviors and peer group affiliation. Data at 54 months, Grade 3 (M age = 9.03, SD = 0.31), Grade 6 (M age = 11.95, SD = 0.34), and Grade 9 (M age = 15.57, SD = 0.78) from the NICHD SECCYD longitudinal investigation of 1,364 participants (52% boys) was analyzed. Overall, results yielded evidence that maternal sensitivity and child temperament at 54 months of age predicted prosocial, aggressive, and delinquent outcomes at age 15. Affiliation with peer groups (especially with prosocial peers) and social behaviors in childhood mediated the aforementioned paths for effortful control, but not for maternal sensitivity. Discussion focuses on the implications for understanding the long-term effects of early childhood predictors on behavioral outcomes in adolescence.