Young children's social and emotional skills (SE skills) are highly predictive of the ease with which they transition to kindergarten as well as their success in the K-12 school system and in life after graduation (Belfield et al., 2015; Blair, 2002; Denham, 2006; Denham et al., 2014; Graziano et al., 2007; Jones et al., 2017; Moffitt et al., 2011; Oberle et al., 2014; Raver & Knitzer, 2002; Trentacosta & Izard, 2007; Zins et al., 2007). Children from low socioeconomic status (SES) backgrounds often enter kindergarten with less well-developed SE skills and lag their peers throughout their school careers (Garcia & Weiss, 2015; Hair et al., 2006; Huffman et al., 2001; Morrissey & Vinopal, 2018; West et al., 2000). Studies of social-emotional learning (SEL) programs indicate that these programs are successful in teaching SE skills, both in school and in after-school programs (ASPs; Corcoran et al., 2018; Durlak et al., 2011; January et al., 2011; Sklad et al., 2012; Taylor et al., 2017). However, less is known about the mechanisms of changes through which SEL programs improve children's SE skills and for whom SEL programs are most beneficial. This dissertation used data collected from WINGS for Kids, a mature, well-developed after-school SEL program attended primarily by African American children from low SES backgrounds in Charleston, S.C., to address the following three sets of questions about the main, moderated and mediated effects of participating in WINGS: (1) whether child and family factors were related to children's participation in WINGS and the direct and indirect relationships between participation and development of SE and self-regulation (SR) skills; (2) whether emotion knowledge acted as a mechanism through which children developed SE and SR skills as a result of taking part in an SEL program; and (3) whether the relationship between taking part in an SEL program and development of SE and SR skills differed based on a variety of child characteristics. The majority of the hypotheses were not supported; however, a few interesting results emerged. The number of family moves, parenting stress, and financial stress predicted children's participation, and kindergarten participation predicted development of SR skills. In 1st grade, gender moderated development of SR skills such that girls benefitted from higher levels of participation as compared to boys. Age moderated teacher-rated bullying behaviors in 1st grade such that relatively older children developed fewer bullying behaviors with higher rates of participation as compared to relatively younger children. In addition, children's development of peer engagement was moderated by their initial levels of peer engagement, such that children with lower peer engagement at the end of kindergarten benefited more from higher rates of participation than children with relatively higher peer engagement. The pattern of largely null results highlights the need for future work that examines fidelity of implementation, including program quality, interrogates the developmental appropriateness of program models, and assures cultural relevance of SEL ASPs for young Black children. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]