We designed the work described in this monograph as a way to advance understanding of the impact of an early childhood curriculum on prekindergarten children’s academic, executive function, self‐regulation, and social outcomes. Our intent was to contribute to an understanding of why past research about the effectiveness of Tools has yielded inconsistent findings. In addition, we use the study of Tools as a case study to help identify challenges faced when trying to understand why curriculum studies, in general, often show limited immediate and long‐term effects on children’s outcomes. In Chapter II, we describe the methodology of the study’s randomized control trial. We also present the results of the primary statistical tests of curriculum effects (i.e., Tools compared with control classrooms) on gains in children’s academic, executive function, self‐regulation, and social skills from prekindergarten to the end of first grade. In Chapter III, we explain the instrument we designed to document the fidelity of implementation of the Tools curriculum. Focusing on teachers who were randomly assigned to implement Tools, we investigate the degree to which the curriculum was delivered as intended and examined the associations between fidelity of implementation and children’s academic, executive function, and social outcomes in prekindergarten. In Chapter IV, we first describe how we collaborated with the developers of Tools to (a) identify and (b) test hypotheses about expected curriculum effects on classroom processes. Included are hypotheses about the amount of non‐instructional behaviors, teacher‐led and child‐directed activities, teacher and child talk, social learning interactions, as well as about the quality of classroom emotional climate, teacher instruction, and children’s level of involvement. As a secondary focus, we investigate whether the identified classroom processes are associated with gains in children’s academic, executive function, self‐regulation, and social skills combining treatment and control classrooms. In Chapter V, we situate the findings from this randomized control trial of the Tools curriculum within the larger context of early childhood education policies and practices. We offer a series of “lessons learned” to help guide future research and policy. (author abstract)