- Schools are increasingly expected to prevent and decrease violence, substance use, and other problem behaviors linked to academics and prepare students to be contributing members of society. One approach with proven success in promoting positive outcomes related to a broad range of student behaviors and personal characteristics is social-emotional learning, also known as positive youth development or social-emotional and character development (SECD) programs. Currently, little is known about the relationship between SECD and these outcomes in low-income, urban, minority populations. The present studies (a) examine key methodological design issues in conducting a cluster-randomized trial with such populations, and (b) assess whether an intervention designed to promote SECD was effective in improving the SECD developmental status of children from this population. Data for this study come from the Chicago cluster-randomized controlled trial (CRCT) of Positive Action (PA). The trial was longitudinal at the school level with a place-focused intent-to-treat design at the student level. This CRCT collected data on children in grades 3 through 8, for a total of 6 years and 8 data collection points. Manuscript #1 focuses on the design, sample, planned analyses, and a latent class analysis (LCA) of mobility patterns. Specifically, the setting and recruitment of schools is described, as well as the process by which schools were matched into pairs and randomized into PA or control, including the list of criteria for school eligibility and variables used for matching. Additionally, this paper thoroughly describes the primary analyses to test for program effects using three-level growth curve models (time nested within students nested within schools), as well as several sensitivity analyses that will also be conducted when evaluating this program. Further, this manuscript discusses secondary tests of meditation and moderation, which will assist in the understanding of how the program works (mediation) and for whom (moderation). Finally, this paper also provides several descriptive statistics and characteristics of the students and teachers in this sample. In terms of baseline equivalency, PA and control schools did not significantly differ on matching variables before or during the trial. Minimal differences were found on baseline reports from students, teachers, and parents; half of these differences favored PA students and half favored control students. Manuscript #2 focuses on the intervention effects on the student-, teacher-, and parent-reported social-emotional outcomes assessed during the Chicago CRCT, following the analytic procedures outlined in the first paper and focusing on the effectiveness of PA on social-emotional outcomes. Results indicate that PA had a significant effect on student self-reports of prosocial interactions, honesty, self-development, self-control, respect for parents and teachers, empathy, altruism, positive actions/feelings, negative moral center, and aggressive problem solving. Additionally, it was found that PA had marginal effects on teacher-reported responsibility. Minimal differences by gender were found; no differences by mobility status were found. Together, these two papers involved a sample of students in a high-risk setting; generating improvements can be particularly difficult in urban areas. The empirical evidence of effectiveness of a SECD program in a high-risk population, as demonstrated in the present study, should serve as a call to action for policymakers and school officials who are increasingly challenged to positively impact not only academic achievement, but also behavior and character development.