Introduction:The purpose of this study was to conduct a meta-analysis investigating the consistency and strength of relations between prosocial behavior, externalizing behaviors, and internalizing symptoms from preadolescence (i.e., 1-9 years) to late adolescence (i.e., 19-25 years). This study directly addresses inconsistencies and gaps in the available literature by providing the field with a detailed, synthesized description of these associations. Method: Fifty-five studies met the inclusion criteria, containing 742 independent correlational effect sizes. Statistical information and other study information was coded and entered into Comprehensive Meta-analysis III software, which was used to analyze results. Results: Results showed that higher levels of prosocial behavior were significantly associated with lower levels of externalizing behaviors, as expected. Additionally, more reported prosocial behavior was related to less reported internalizing symptoms. Follow-up analyses revealed specific relationships between prosocial behavior and aggression, deviant peer association, risky sexual behavior, substance use, delinquency/general externalizing behavior, depression, and general internalizing behaviors (i.e., emotional problems, negative emotionality). A variety of moderators of these associations were considered, including age and sex. Conclusions: Findings are discussed in the context of the broader research literature, weaknesses in the field are noted, and numerous meaningful directions for future research are presented.Prosocial behavior is defined as voluntary behavior intended to benefit others and comes in many types, such as comforting, sharing, volunteering, donating, and offering physical or emotional assistance to others, etc. (Eisenberg, Spinrad, & Knafo-Noam, 2015). Prosocial behavior is relevant to adolescents, whose prosocial opportunities expand (Padilla-Walker, Dyer, Yorgason, Fraser, & Coyne, 2015a), and who are especially sensitive to the quality and types of interactions they have with those in their social networks (e.g., parents and peers, Allen et al., 2003;Masten et al., 2009). In light of the developmental significance of prosocial behavior, a growing body of recent research identifies prosocial behavior as protective against myriad problematic outcomes, including but not limited to aggression (