Few instruments provide reliable, valid data on peer-behavioral indicators of risk and competence in young children. The authors developed a teacher-report measure of young children's behavior with peers at school—the Child Behavior Scale (CBS)—and evaluated its reliability and validity. Data were gathered on 2 cohorts (n = 206 per cohort) of 5- to 6-year-old children; teachers rated children on Aggressive With Peers, Prosocial With Peers, Asocial With Peers, Excluded by Peers, Anxious–Fearful, and Hyperactive–Distractible subscales twice during the school year. Scores were internally consistent, distinct, and relatively stable over time. The validation paradigm produced a network of correlations that was, overall, consistent with the hypothesized conceptual structure of the CBS. These findings replicated across cohorts and provide sufficient evidence of the reliability and validity of the CBS to recommend its use for behavioral assessment with young children. (APA PsycInfo Database Record (c) 2019 APA, all rights reserved)