Research Findings: This study examines lexical- and sentence-level dimensions of academic language to describe teachers' natural use of academic language and its association with vocabulary growth in 489 at-risk 4-year-olds enrolled in Head Start preschool classrooms. Using transcripts derived from video recordings of book-reading sessions in 52 classrooms, we developed measures to assess the relationships among teachers' use of lexical elements (amount, sophistication, and diversity of vocabulary) and utterance length (mean length of utterance in words) and children's end-of-year receptive vocabulary scores. Hierarchical linear models indicated that children in classrooms where teachers used more lexical elements had higher end-of-year receptive vocabulary scores than those who heard less. Conversely, children in classrooms where shorter utterances were used had higher end-of-year receptive vocabulary scores. Classroom factors also played a role in children's vocabulary scores, indicating a need to address classroom environments in addition to teachers' language use. Practice or Policy: Results indicate that teachers should consider the unique contributions of lexical- and sentence-level elements when providing vocabulary instruction for children with below-the-mean vocabulary scores. Implications for instruction are addressed.