The Collaborative for Academic, Social, and Emotional Learning (CASEL) launched the Collaborating Districts Initiative (CDI) with the goal of supporting school districts to build their capacities to promote social and emotional learning (SEL) for all students. Twenty large, urban districts across the country participate in the CDI. Although many approaches to support SEL start in the classroom, the CDI model takes an ecological approach and addresses the district-level systems, school-level practices, and classroom-level SEL instruction as three necessary components of effective implementation and sustainability. This study investigates the following research questions: (1) How common are SEL implementation strategies in schools nationally, and how do they vary by school characteristics? and (2) Is there a higher level of systemic SEL implementation among CDI districts compared with demographically similar non-CDI districts? If yes, in which areas? To answer these questions, CASEL engaged RAND researchers to learn whether the 20 districts' participation in the CDI has resulted in more frequent implementation of SEL practices than in peer districts and to learn about SEL implementation nationally. Accordingly, they fielded a survey to groups of K-12 classroom teachers and principals about their schoolwide SEL practices and strategies in the 2021-2022 school year. This report begins by explaining the CDI initiative in greater detail and then presents the nationally representative survey results. Next, it explores ten indicators of schoolwide SEL implementation in the CDI districts and in comparable non-CDI districts. The report concludes with a discussion of the implications for districts nationally and, specifically, for CDI districts and CASEL.