This issue brief is part of a five-part series that reports findings from research conducted by ACT in collaboration with Region One Education Service Center. This brief focuses on the efficacy of SEL lessons implemented during the 2019-2020 school year and examines their relationship to school-reported student outcomes such as grades, attendance, and conduct. The goals of the study were to better understand the effects of SEL programming on student outcomes and to explore other factors related to efficacy, such as lesson dosage and fidelity of implementation. Results of the study suggest time and effort are required to provide the sustained implementation of programming that may be necessary to produce long-term benefits to students. Findings from the current study also underscore the importance of a structured implementation plan and continued monitoring of the fidelity and dosage of implementation. [For Brief 2 in the series, see ED616538. For the Executive Summary, see ED616534.]