Educators and researchers are increasingly interested in evaluating and promoting socio-emotional learning (SEL) beginning in early childhood (Newman & Dusunbury in 2015; Zigler & Trickett in American Psychologist 33(9):789-798 https://bibliotheek.ehb.be:2102/10.1037/0003-066X.33.9.789, 1978). Decades of research have linked participation in high-quality early childhood education (ECE) programs (e.g., public prekindergarten, Head Start) to multidimensional wellbeing. ECE programs also have demonstrated potential to be implemented at large scales with strong financial returns on investment. However, relatively few studies have investigated the effects of ECE programs on SEL, particularly compared to smaller-scale, skills-based SEL interventions. Furthermore, among studies that have examined SEL, there is a general lack of consensus about how to define and measure SEL in applied settings. The present paper begins to address these gaps in several ways. First, it discusses conceptual and methodological issues related to developmentally and culturally sensitive assessment of young children's socio-emotional functioning. Second, it reviews the empirical research literature on the impacts of three types of early childhood programs (general prekindergarten programs; multi-component prekindergarten programs; and universal skills-based interventions) on SEL. Finally, it highlights future directions for research and practice.