As public policy attention has focused on families’ access to preschool, both federal and state policymakers are concerned with the quality of public preschool. Discerning the components of preschool quality, measured by how much children learn, is therefore an important task for early education researchers. Whether and how preschool class composition, and in particular, racial/ethnic diversity, may affect children’s learning has important policy implications for the expansion of early childhood education. Employing a multi-level analysis, this paper explores the extent to which empirical data provides evidence of racial/ethnic diversity within 11 state pre-K classrooms, whether racial/ethnic composition was associated with children’s language development during preschool, whether the racial and socio-economic composition of classrooms interacted to influence their growth in language skills, and whether aspects of classroom quality explained any relationship between racial/ethnic composition and children’s learning. The results indicate that the racial and socio-economic compositions of pre-K classrooms represent potentially important and independent components of preschool quality. Both measures of classroom composition were reliable predictors of children’s language learning, even with the inclusion of a measure of instructional quality. The findings have important policy implications for states that seek to expand access to high-quality preschool.