Although research has demonstrated that both parents and peers influence adolescent development, it is not clear whether these relationships also serve as contexts for emotion socialization. In the current longitudinal study, we investigated whether maternal and peer emotion socialization were related to adolescent girls' daily emotions, emotion regulation, and social and emotional adjustment. The sample included 160 adolescent girls from low-income families followed across 2 years. At Time 1 (T1), girls reported on maternal and peer emotion socialization practices during laboratory visits. At Time 2 (T2), girls reported on daily negative and positive affect using ecological momentary assessment across 2 weeks. Emotion regulation, internalizing problems, and prosocial behavior were assessed during laboratory visits at both T1 and 2 years later (Time 3 [T3]). Results demonstrated that higher levels of maternal and peer emotionally supportive socialization practices were associated with lower levels of girls' daily negative affect. Mothers' supportive practices also predicted increases in girls' emotion regulation over time. Both maternal and peer unsupportive practices predicted more internalizing problems, and peer unsupportive practices predicted less prosocial behavior over time. This study supports and expands Eisenberg's heuristic model by demonstrating that both maternal and peer emotion socialization are associated with adolescent girls' emotional and behavioral outcomes, and maternal and peer emotion socialization have differential effects.