Research Findings: This study examined associations between the expressive vocabulary of classroom peers and children's own vocabulary knowledge using conceptual scoring in a linguistically diverse sample of 4-year-olds who attended universal preschool programs in a metropolitan area. Higher peer conceptually scored expressive vocabulary was associated with higher conceptually scored expressive vocabulary for all children. The association was stronger for children who were dual language learners (DLLs), such that higher peer ability was associated with stronger growth in conceptually scored expressive vocabulary for DLLs across languages than for children fluent in English. Practice or Policy: This research suggests that policymakers may want to investigate the advantages of setting aside slots in targeted programs for children who have stronger conceptual vocabularies. Administrators should consider conceptual and verbal ability and classroom composition when assigning children to classes. This research also suggests that greater use of intentional pairs or small groups and strategies such as coaching peers in supporting one another's conceptual vocabulary may be particularly important for DLLs. (author abstract)