Classroom-based physical activity (CBPA) is an emerging strategy for increasing youth physical activity, as well as improving academic achievement and on-task behavior. This paper describes the rationale, development, implementation, and evaluation of a professional development training designed for classroom and physical education teachers to increase implementation of CBPA in elementary schools. Twenty-two 5th grade classroom and 12 physical education teachers participated in the training, which was based on key principles of adult learning, including modeling, active learning, and feedback and reflection. The training was also designed to intentionally address known implementation barriers, such as insufficient resources, concerns about detracting from academics' and teachers' comfort with teaching with physical activity. Immediate post training evaluations showed that the training was very well received by both classroom and PE teachers. Positive feedback received after teachers implemented the CBPA strategies in their classrooms further reinforced the premise that engaging and relevant professional development is necessary to influence teachers' use of CBPA.