For decades, psychological research has examined the extent to which children's and adolescents' behavior is influenced by the behavior of their peers (i.e., peer influence effects). This review provides a comprehensive synthesis and meta-analysis of this vast field of psychological science, with a goal to quantify the magnitude of peer influence effects across a broad array of behaviors (externalizing, internalizing, academic). To provide a rigorous test of peer influence effects, only studies that employed longitudinal designs, controlled for youths' baseline behaviors, and used "external informants" (peers' own reports or other external reporters) were included. These criteria yielded a total of 233 effect sizes from 60 independent studies across four different continents. A multilevel meta-analytic approach, allowing the inclusion of multiple dependent effect sizes from the same study, was used to estimate an average cross-lagged regression coefficient, indicating the extent to which peers' behavior predicted changes in youths' own behavior over time. Results revealed a peer influence effect that was small in magnitude (β¯ = .08) but significant and robust. Peer influence effects did not vary as a function of the behavioral outcome, age, or peer relationship type (one close friend vs. multiple friends). Time lag and peer context emerged as significant moderators, suggesting stronger peer influence effects over shorter time periods, and when the assessment of peer relationships was not limited to the classroom context. Results provide the most thorough and comprehensive synthesis of childhood and adolescent peer influence to date, indicating that peer influence occurs similarly across a broad range of behaviors and attitudes. (PsycInfo Database Record (c) 2021 APA, all rights reserved).