This study is part of a larger randomized efficacy trial examining the impact of Behavioral, Emotional, and Social Training: Competent Learners Achieving School Success (BEST in CLASS), a Tier 2 intervention that targets the prevention of emotional/behavioral disorders in young, high risk children. In this investigation, we examined teachers' implementation and maintenance of instructional practices in early childhood classrooms and the corollary relationships between teacher implementation of the specific instructional practices associated with BEST in CLASS and child engagement and problem behaviors. Fifty-three teachers (26 in treatment and 27 in comparison) and 130 preschool-aged children (66 in treatment and 64 in comparison) participated. Findings indicated that teachers' who received training and coaching in the BEST in CLASS intervention increased their use of specific instructional practices in comparison with teachers in the control condition. In addition, children whose teachers' received the BEST in CLASS intervention demonstrated increased engagement and a decrease in problem behaviors in comparison with those children who were in the control group. Positive teacher-child interactions increased and negative teacher-child interactions decreased in the intervention group in comparison with the control group. Results are discussed in relation to measuring teachers' implementation of instructional practices and implementation science. (author abstract)