Social and emotional learning (SEL) is the process by which children and adults learn to understand and manage emotions, maintain positive relationships, and make responsible decisions. This is the first in a series of four related reports about what is known about SEL programs for students ages 3-8. The report series addresses four issues raised by the Regional Educational Laboratory (REL) Mid-Atlantic's Early Childhood Education Research Alliance: characteristics of effective SEL programs (part 1), implementation strategies and state and district policies that support SEL programming (part 2), teacher and classroom strategies that contribute to social and emotional learning (part 3), and outcomes of social and emotional learning among different student populations and settings (part 4). This report identifies key components of effective SEL programs and offers guidance on selecting programs. The following are appended: (1) Literature search methodology and coding results; (2) Publications included in the review; and (3) Additional resources for selecting a social and emotional learning program. [For part 2, "A Review of the Literature on Social and Emotional Learning for Students Ages 3-8: Implementation Strategies and State and District Support Policies (Part 2 of 4). REL 2017-246," see ED572722. For part 3, "A Review of the Literature on Social and Emotional Learning for Students Ages 3-8: Teacher and Classroom Strategies that Contribute to Social and Emotional Learning (Part 3 of 4). REL 2017-247," see ED572723. For part 4, "A Review of the Literature on Social and Emotional Learning for Students Ages 3-8: Outcomes for Different Student Populations and Settings (Part 4 of 4). REL 2017-248," see ED572724.]