This study explores whether the quality of Head Start teachers' language fosters gains in children's vocabulary, literacy, and math skills, and whether the pattern is similar for low income English language learners and English speakers. Children (N=191) attended two urban Head Start Centers. The CLASS (language modeling scale) was used to observe the quality of teachers' language. Children's skills were assessed in the fall and spring on measures of expressive and receptive vocabulary, early literacy, and math skills. The pattern of results differed for English language learners and English speakers and across outcome measures. The quality of teachers' language predicted gains in English language learners' receptive and expressive vocabulary, but not that of English speakers. In addition, the receptive vocabulary of the English language learners predicted gains in their phonological awareness and math skills. The relation between teachers' language and children's print knowledge was moderated by their receptive vocabulary. That is, the quality of teachers' language predicted gains in print knowledge only among those children who had higher vocabulary scores. These results underscore the role that teachers' language can play and the importance of children's vocabulary for their early academic development. (author abstract)