The prevention science approach to emotional/behavioral disorders (EBD) focuses on early intervention targeting risk and resilience factors (e.g., early problem behaviors, teacher-child interactions, classroom climate). The current study investigates the effectiveness of BEST in CLASS, a classroom-based indicated preventive intervention targeting young children at risk for EBD, in terms of producing clinically meaningful reductions of problem behaviors and improvements in social skills. A total of 185 early childhood teachers (92 in the treatment condition) and 462 young children (230 in treatment) participated in the study. Teachers completed rating scale measures of problem behaviors and social skills. The clinical significance of the findings indicated that the BEST in CLASS condition had a lower percentage of children with clinical or borderline range scores at posttest when compared with children in the control classrooms. The BEST in CLASS prevention model, clinically significant intervention outcomes, and prevention of EBD are discussed. (author abstract)