In a randomized control study, Head Start teachers were assigned to either an intervention group that received intensive, ongoing professional development (PD) or to a comparison group that received the "business as usual" PD provided by Head Start. The PD intervention provided teachers with conceptual knowledge and instructional strategies that support young children's development of vocabulary, alphabet knowledge, and phonological sensitivity. Results indicated that, after 1 academic year, teachers in the intervention group created higher quality classroom environments, as measured by the Early Language and Literacy Classroom Observation (M. W. Smith, D. K. Dickinson, A. Sangeorge, & L. Anastasopoulos, 2002) and Classroom Assessment Scoring System (R. C. Pianta, K. M. La Paro, & B. K. Hamre, 2007), and by videotapes of their classroom book readings. Further, children in the intervention group performed significantly better than comparison-group peers on measures of receptive vocabulary and phonological sensitivity but showed equivalent alphabet learning. Moreover, variation in classroom quality and fidelity to the intervention were linked to child outcomes, illuminating which particular aspects of teachers' improved practices were linked to children's gains. Findings provide new details about the mechanisms through which intensive and intentional PD can enhance Head Start teachers' classroom practices and, by extension, improve Head Start children's language and preliteracy outcomes. (Contains 8 tables.)