This paper presents the process evaluation model for the physical activity intervention component of the Child and Adolescent Trial for Cardiovascular Health (CATCH) and describes the major procedures used to monitor CATCH PE, the physical education intervention. The paper focuses on CATCH PE teacher training and in-service support as well as on the curriculum implementation. Monitoring training and support included assessing the in-service training workshops and the follow-up on-site assistance provided by staff. Monitoring the implementation included assessing the quantity and quality of CATCH PE instruction in terms of student physical activity engagement and lesson context, the fidelity of the curricular implementation, and the opportunities for other physical activity by children throughout the school day.