This study explored whether the quality of Head Start teachers' language improves the vocabulary, literacy, and math skills of English language learners and English speakers. 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 in English speakers. In addition, the receptive vocabulary of the English language learners predicted gains in their phonological awareness and math skills. The quality of teachers' language predicted gains in print knowledge for children who had higher vocabulary scores. These results show the importance of teachers' language for children's vocabulary and early academic development. (author abstract)