This study examined associations between peers' expressive language abilities and children's development of receptive and expressive language among 1,812 four-year olds enrolled in 453 classrooms in 11 states that provide large-scale public pre-kindergarten (pre-k) programs. Higher peer expressive language abilities were positively associated with children's development of receptive and expressive language during pre-k. The positive association between peers' expressive language abilities and children's receptive language development was stronger for children who began pre-k with higher receptive language skills and within classrooms characterized by better classroom management. Implications of these findings for understanding ecological inputs to children's language development and for designing effective pre-k programs are discussed.