To date, there have been few causally interpretable evaluations of the impacts of preschool curricula on the skills of children at-risk for academic difficulties, and even fewer studies have demonstrated statistically significant or educationally meaningful effects. In this cluster-randomized study, we evaluated the impacts of a literacy-focused preschool curriculum and two types of professional development on the emergent literacy skills of preschool children at-risk for educational difficulties. Forty-eight preschools were randomly assigned to a business-as-usual control, a literacy-focused curriculum with workshop-only professional development, or a literacy-focused curriculum with workshop plus in-class mentoring professional development conditions. An ethnically diverse group of 739 preschool children was assessed on language and literacy outcomes. Results revealed significant and moderate effects for the curriculum and small, mostly nonsignificant, effects of professional development across child outcomes and classroom measures.