A population-based randomized intervention trial for the prevention of conduct problems (i.e., oppositional defiant disorder and conduct disorder) is described. The LIFT (Linking the Interests of Families and Teachers) intervention was designed for all first- and fifth-grade elementary school boys and girls and their families living in at-risk neighborhoods characterized by high rates of juvenile delinquency. The 10-week intervention strategy was carefully targeted at proximal and malleable antecedents in three social domains that were identified by a developmental model of conduct problems. From 12 elementary schools, 671 first and fifth graders and their families participated either in the theory-based universal preventive intervention or in a control condition. The intervention consisted of parent training, a classroom-based social skills program, a playground behavioral program, and systematic communication between teachers and parents. A multiple measure assessment strategy was used to evaluate participant satisfaction and participation, fidelity of implementation, and the immediate impacts of the program on targeted antecedents.