Many high school classrooms in the United States have physics teachers who are teaching the subject outside their certified content areas because of a severe shortage of physics teachers. There is a critical need to improve the knowledge base of these out-of-field teachers. This comparative case study examined the instructional practice of Advanced Placement (AP) physics teachers who participated in a 3-year physics focused professional development and compared them with other AP physics teachers who did not participate in the training. Quantitative and qualitative data were collected from classroom observations, surveys, teacher interviews, and observer's field notes. Results indicate that the teachers who experienced the training had (a) large treatment effect size difference in their instructional practice, (b) changes in their self-efficacy, and (c) observable improvement in their pedagogical content knowledge. The result represents initial empirical findings of the impact of the targeted training on the knowledge base and instructional practice of the AP physics teachers.