Achieving high-quality preschool at scale is challenging; to do so likely entails a combination of program standards, teacher qualifications and compensation, on-site quality monitoring, and professional development (PD). This study aims to examine the impact of investments in PD within the context of an expansion of universal preschool in one of the nation’s largest school districts. We leverage the opportunity provided by a “natural experiment” to estimate PD’s effects that embeds an evidence-based math curriculum in interdisciplinary units of study with coaching support on teacher math practices. A total of 95 schools participated in this study (51 treatment and 44 comparison schools). Treatment sites implemented more teacher-led math activities for a longer period compared to control sites. The size and magnitude of the impacts of a curriculum and PD program implemented at scale were comparable to results from studies of small-scale efficacy trials. (author abstract)