The Social and Emotional Learning Challenge was designed to identify promising practices for building skills in six areas: emotion management, empathy, teamwork, initiative, responsibility, and problem solving. The Challenge was a partnership between expert practitioners delivering exemplary programs in eight unique communities, a team of researchers, and a national foundation. This technical report includes a discussion of theory and methodology used to produce the SEL Challenge findings. This technical report is written for agency leaders, evaluators, funders, consultants, and policy makers who want to assess the validity of the SEL Challenge findings or mount their own local Challenges. [The SEL Challenge was made possible by the financial support of the Susan Crown Exchange (SCE). This report was prepared by the Forum for Youth Investment's David P. Weikart Center for Youth Program Quality.]