There is surprisingly little empirical research examining issues of fidelity of implementation within the early childhood education literature. In the MyTeachingPartner project, 154 teachers were provided with materials to implement a supplemental classroom curriculum addressing six aspects of literacy and language development. The present study examines the degree of variability in three aspects of implementation fidelity--dosage, adherence, and quality of delivery--and whether these components of fidelity were associated with children's growth in language and literacy skills across the preschool year. Findings indicate that teachers reported using the curriculum fairly often (dosage) and that they were observed to generally follow curricular lesson plans (adherence). In contrast, the quality of delivery, defined as the use of evidence-based teacher-child interactions for teaching literacy and language, was much lower. Children in classrooms in which activities were observed to last for longer (dosage) and in which teachers exhibited higher quality of delivery of literacy lessons made significantly greater gains in early literacy skills across the preschool year. Also, teachers' use of higher quality language interactions was associated with gains for children who did not speak English at home. Results have implications for teacher professional development and the supports provided to ensure that curricula are delivered most effectively. (Contains 4 tables and 2 figures.)