This is the technical appendix to a report that offers early lessons from an initiative focused on social and emotional learning (SEL) in elementary schools and out-of-school time (OST) programs. The main report is available at www.rand.org/t/RRA379-1. In 2016, in an effort to gain knowledge about how to help children develop SEL skills, The Wallace Foundation launched a six-year project called the Partnerships for Social and Emotional Learning Initiative (PSELI). Wallace selected six communities—Boston, Massachusetts; Dallas, Texas; Denver, Colorado; Palm Beach County, Florida; Tacoma, Washington; and Tulsa, Oklahoma—to explore whether and how children benefit when schools and OST programs partner to improve and align SEL, as well as what it takes to do this work. The findings and lessons outlined in the main report are based on these six communities' experiences implementing SEL for elementary school–aged students during the first two years of PSELI. This technical appendix provides a description of the study; details on methods for interviews, staff surveys, class observations, and document collection; and study limitations. (author abstract)