This study aimed to understand how relationships with peers and teachers contribute to the development of internalizing problems via children's social self-concept. The sample included 570 children aged 7 years 5 months (SD = 4.6 months). Peer nominations of peer rejection, child-reported social self-concept, and teacher-reported internalizing problems were assessed longitudinally in the fall and spring of Grades 2 and 3. Teacher reports of support to the child were assessed in Grade 2. Results showed that peer rejection impeded children's social self-concept, which in turn affected the development of internalizing problems. Partial support was found for individual (but not classroom-level) teacher support to buffer the adverse effects of peer problems on children's self-concept, thereby mitigating its indirect effects on internalizing problems. © 2013 Society for Research in Child Development, Inc.