In 2016, in an effort to gain knowledge about how to help children develop social and emotional learning (SEL) skills, The Wallace Foundation launched a six-year project called the Partnerships for Social and Emotional Learning Initiative (PSELI). The goals of PSELI are for students to experience reinforcing messages about SEL both in school and in out-of-school time (OST) programs; practice social and emotional skills in both settings; and experience consistent, supportive relationships between adults and students. To achieve these goals, school districts and out-of-school time intermediaries (OSTIs) have partnered to develop professional development (PD) about SEL for school and OST staff, help elementary schools and their OST partners develop closer working relationships, and implement reinforcing SEL practices and instruction across both settings. In what the authors believe is the most-comprehensive SEL implementation study to date, they draw lessons that can help school districts and OST providers carry out their own SEL programs. [For the technical appendix, see ED609666.]