Students' early school experiences have a significant and long-term effect on key academic and social/behavioral outcomes. Evidence-based programs that both increase the quantity and quality of positive teacher-student interactions and decrease the frequency of negative teacher-student interactions may be critical for changing negative patterns and improving students' early learning environments. The purpose of this article is to describe the iterative development of one such program, BEST in CLASS--Elementary, which has been adapted from an evidence-based program designed to reduce risk for emotional/behavioral disorders by improving teachers' interactions and relationships with preschool-age children with chronic problem behavior. Following a brief description of the parent program, 4 overlapping phases of intervention adaptation and development are described. Future directions for the intervention and applications of this iterative process to intervention development work and research are discussed.