This study examined teacher language use in Head Start classrooms (N = 43) from the perspective of the Systemic Linguistics Approach (SLA) to describe the nature of teacher support for children's acquisition of academic language and factors that shape language use. Using a sample of teachers who were part of a larger study on early language/literacy curricula, we hypothesized that evidence of emergent academic language registers might be identified using utterance-level descriptions of language and that language use would vary across the three settings examined: Book Reading, Group Content Instruction, and Small Group Instruction. Differences in overall patterns of language were also expected to relate to teachers' pedagogical skill and the intervention condition to which they were exposed in the larger study. Language use within setting was expected to vary by the content of instruction and, in Book Reading, the books being read. These hypotheses were examined using a corpus of 146,000 teacher utterances from a study in Head Start pre-kindergarten classrooms that included a business-as-usual condition and two intervention conditions. Language variables included use of sophisticated vocabulary, diversity of words used, number of words used, and syntactic complexity; semantic content variables included talk about vocabulary, concepts, and skills. We found evidence of emergent academic registers in Book Reading, Group Content Instructional Time and Small Group Instruction; differences in teacher talk were associated primarily with setting, and few differences related to teacher pedagogical skill or intervention condition. Language use during Book Reading was affected by the type of book read. Our findings identify factors that should be considered when planning interventions and studying classroom language. (author abstract)