This study evaluated effects of Positive Action (PA), a school-based social-emotional and character development program, on self-esteem levels and processes among minority, low-income, urban youth. A matched-pair, cluster-randomized controlled trial was conducted in 14 Chicago Public Schools with outcomes assessed longitudinally for a cohort of youth followed from Grades 3 to 8. A total of 1,170 students participated in the study (53% female, 48% African American, and 27% Hispanic). Students in PA schools had more favorable change and endpoint scores on indices of self-esteem in the domains of peer and school and use of both adaptive and (to a lesser extent) maladaptive processes for developing and maintaining self-esteem. These results align with areas of emphasis within the PA program and illustrate how important areas of impact on self-esteem-related outcomes may be overlooked without differentiated assessments of both self-esteem levels and processes. [The initial phase on this project (R305L030072), a component of the Social and Character Development (SACD) Research Consortium, was a collaboration among IES, the Centers for Disease Control and Prevention's (CDC) Division of Violence Prevention, Mathematica Policy Research Inc. (MPR), and awardees of SACD cooperative agreements (Children's Institute, New York University, Oregon State University, University at Buffalo--SUNY, University of Maryland, University of North Carolina--Chapel Hill, and Vanderbilt University).]