Scientifically-validated curricula, which demonstrate positive child outcomes, are the heart of high-quality preschools. The present study examines the impact of supplementing the whole-child comprehensive HighScope curriculum with the Numbers Plus Preschool Mathematics curriculum, training, and coaching (hereafter, Numbers Plus), using a multi-site cluster randomized trial (MSCRT) consisting of four levels: children, classrooms, sites, and states. The study is the first effort to examine the effect of supplementing a widely used comprehensive whole-child curriculum with a newly developed skill-based curriculum, where both were created by the same developer. Twenty-two sites, 52 classrooms, and 583 children (323 treatment; 260 control) in three states (Michigan, Maine, and Ohio) were selected to participate in the study. The findings show promise in Numbers Plus as an evidence-based, skill-based supplement to the whole-child comprehensive HighScope curriculum. The frequency of Numbers Plus small-group times, topics covered, and coaching contacts reached the intended dosage, indicating feasibility of implementation. In addition, while all children increased their mathematics knowledge and skills during their preschool year, we found an additional positive effect of Numbers Plus on children's math outcomes. However, the effect size was small. The study provides future directions for developers of a comprehensive whole-child curricula like HighScope to ensure evidence-based curriculum improvement and development. (author abstract)