The effects of training and supervision on instructor knowledge and performance of discrete trial teaching (DTT) within three domains (DTT Technical Skills; Work Session Preparation/Conclusion; and Student Engagement/Management) were examined in this study. Eight undergraduate student instructors received an 8-hr training in DTT and support skills accompanied by a pre- and post-test of knowledge. The instructors then taught a variety of skills to six students with autism in a community-based preschool, where instructor competence was tracked and performance feedback provided using the Discrete Trial Teaching Competency Checklist for Instructors. Competence in all three domains improved over time with performance feedback. However, significant variability was observed within and between instructors, and performance in some areas remained below optimal levels even with regular supervision and performance feedback. Implications for training and supervising instructors to implement DTT with children with autism in community-based settings are discussed.