Research Findings: The aim of the current study was to examine the extent to which the effects of preschool classroom quality on children's mathematics is moderated by executive functioning (EF) during the preschool year. The study sample included 102 children (M age = 53.57 months [SD = 5.42]; 49% male) and one of their parents recruited from 26 Head Start or center- and community-based preschools. Results from main effects analyses indicated that one indicator of quality, emotional support, was significantly associated with children's mathematics skills. Further, interactions between the quality domains and EF were significant, indicating that children with higher levels of EF benefited substantially more in terms of their mathematics skills if they were in classrooms with higher quality. Practice or Policy: These findings have implications for intervention work designed to support school readiness and for policies regarding the provision of high quality early care and education.