We examined the amount of preschool lead/assistant teachers’ English/Spanish language use and relations between quality of teacher-child interactions, and Dual Language Learners’ (DLLs) English/Spanish bilingual vocabulary in 31 English-medium Head Start classrooms. Measures in this study included (a) children’s conceptual English/Spanish bilingual receptive and expressive vocabulary, (b) classroom quality via systematic observation, (c) video coding of the amount of utterances of teacher Spanish use in classrooms, and (d) a set of child- and teacher-level covariates. Results showed that Spanish was used much less than English by both teachers and DLLs. DLLs spoke more Spanish than teachers and directed most Spanish to peers. For high levels of Emotional Support, teachers’ Spanish use was significantly negatively associated with English/Spanish bilingual expressive and receptive vocabulary. For low levels of Emotional Support and high levels of Instructional Support, higher amounts of teachers’ Spanish use were significantly positively associated with higher levels of children’s English/Spanish bilingual expressive and receptive vocabulary. (author abstract)