Roger P. Weissberg, Distinguished Emeritus Professor of Psychology at the University of Illinois at Chicago and Chief Knowledge Officer of the Collaborative for Academic, Social, and Emotional Learning (CASEL), which he helped found in 1994, passed away in 2011 after a long illness. Weissberg’s pioneering scholarship and passionate advocacy were instrumental in establishing social and emotional learning (SEL) as an evidence-based, scalable, and transformative approach to promoting positive youth development. Over the course of 40 years, Weissberg’s collaborative research generated strong evidence for the short- and long-term benefits of school-based SEL programs and interventions. With the co-founding of the Coalition for Academic, Social and Emotional Learning (CASEL), Weissberg and his collaborators established an SEL Framework that has continued to evolve, create a resource clearinghouse that shares evidence-based SEL curriculum and intervention strategies and resources, and transform local, state, and national educational policy and practice to make SEL an integral part of students’ education experiences from preschool through high school. This chapter explores Weissberg’s legacy and research contributions, highlighting how his work has shaped the field of SEL and continues to influence educational practices and policies.