Six years ago the Collaborative for Academic, Social, and Emotional Learning (CASEL) took the unprecedented step of launching an effort to study and scale high-quality, evidence-based academic, social, and emotional learning in eight of the largest and most complex school systems in the country: Anchorage, Austin, Chicago, Cleveland, Nashville, Oakland, Sacramento, and Washoe County, Nevada. With the recent addition of Atlanta and El Paso, the Collaborating Districts Initiative (CDI) now includes 10 districts, enrolling 900,000 students a year. It is one of the most comprehensive and ambitious school district improvement initiatives ever. The goal of the CDI was to create a comprehensive shift in how superintendents and entire school districts approach education. In order to help redefine quality education (beyond test scores alone), and to prioritize the practices in classrooms, schools, and communities for promoting the social and emotional development of children, the CDI focused on systemic social and emotional learning (SEL) implementation. This report describes why SEL is needed, highlights seven key insights drawn from CASEL's experience over the last six years, and describes the impact of the CDI's efforts. It concludes with CASEL's goals for the future.