Research has consistently linked early problem behavior with later adjustment problems, including antisocial behavior, learning problems and risk for the development of emotional/behavioral disorders (EBDs). Researchers have focused upon developing effective intervention programs for young children who arrive in preschool exhibiting chronic problem behaviors; however, Tier-2 interventions that can be delivered by teachers with fidelity in authentic settings are lacking. This study examined the effect of BEST in CLASS, a Tier-2 intervention delivered by teachers, on child problem behavior, teacher-child interactions and teacher-child relationships using a cluster randomized controlled trial design. Participants were 465 children (3-5 year olds) identified at risk for the development of EBDs and their 185 teachers from early childhood programs located in two southeastern states. Significant effects were found across both teacher reported (ES ranging from 0.23 to 0.42) and observed child outcomes (ES ranging form 0.44-0.46), as well as teacher-child relationships (ES ranging from 0.26 to 0.29) and observed teacher-children interactions (ES ranging from 0.26 to 0.45), favoring the BEST in CLASS condition. Results suggest the promise of BEST in CLASS as a Tier-2 intervention for use in authentic early childhood classroom contexts and provide implications for future research on transactional models of teacher and child behavior. (author abstract)