Effectiveness studies of preschool social–emotional programs are needed in low-income, diverse populations to help promote the well-being of at-risk children. Following an initial program efficacy study 2 years prior, 248 culturally diverse Head Start preschool children participated in the current effectiveness trial and received either the Emotion-Based Prevention Program (EBP) or the I Can Problem Solve (ICPS) intervention. Pre- and postdata collection included direct child assessment, teacher report, parent interview, and independent observations. Teachers implementing the EBP intervention demonstrated good and consistent fidelity to the program. Overall, children in EBP classrooms gained more emotion knowledge and displayed greater decreases in negative emotion expressions and internalizing behaviors across the implementation period as compared to children in ICPS classrooms. In addition, cumulative risk, parental depressive symptoms, and classroom climate significantly moderated treatment effects. For children experiencing more stress or less support, EBP produced more successful outcomes than did ICPS. These results provide evidence of EBP sustainability and program effectiveness, as did previous findings that demonstrated EBP improvements in emotion knowledge, regulation skills, and behavior problems replicated under unsupervised program conditions.