A growing body of research exists on the effectiveness of classroom-based intervention programs to prevent and ameliorate social, emotional, and learning difficulties demonstrated by young children at risk for emotional and behavioral disorders (EBD). Yet, little research has examined the influence of these targeted intervention programs on the teachers who are trained to deliver them. Impacts of the professional development associated with the intervention on teachers who implement the intervention are important to examine. Data from a 4-year study examining the efficacy of BEST in CLASS were used to examine the effect of BEST in CLASS on teachers' implementation of effective instructional practices, their sense of self-efficacy, and classroom quality. Using a multisite cluster randomized trial, a total of 186 early childhood teachers were included (92 assigned to BEST in CLASS and 94 assigned to a comparison group). Findings indicate BEST in CLASS positively impacted teachers' use of effective instructional practices, their sense of self-efficacy, and their overall classroom quality compared to teachers in the control condition. Future research and implications for professional development are discussed.