Getting Ready for School (GRS) is a new school readiness intervention for teachers and parents, designed to help children develop early literacy, math, and self-regulation skills. GRS was implemented in 19 Head Start classrooms. In the present study we examined variability in different aspects of intervention fidelity including dosage, adherence, and child engagement. In addition, we studied the association among classroom, teacher and student characteristics and fidelity, and whether measures of fidelity were associated with children's growth in math, early literacy, and self-regulation skills across the preschool year. Findings indicate that on average teachers reported completing almost 80% of the activities assigned, and that they were observed to adhere fairly well to the lessons. Child engagement was observed to be moderate to high across classrooms. Classroom quality, as measured by the CLASS, and age of children were positively associated with adherence. Teachers that had participated in GRS for two years were more likely to complete more activities. Different components of fidelity were associated with child outcomes. Percentage completion of math and literacy activities were positively associated with growth in math and literacy skills. Children in classrooms in which teachers adhered more faithfully to the curriculum made significantly greater gains in literacy, math, and self-regulation skills. Child engagement was positively associated with a measure of executive function. Results highlight the importance of examining implementation fidelity. Implications for preschool teachers are discussed. (author abstract)