Health professionals are faced with the growing challenge of addressing childhood overweight. Few overweight prevention efforts have targeted young children, particularly children in child care settings. We describe the theory and development of a novel nutrition and physical activity environmental intervention. On the basis of findings from interviews and focus groups, a review of national recommendations and standards, and a review of the literature, we developed a nutrition and physical activity environmental self-assessment instrument to assess physical activity and nutrition policies and practices in child care settings. An intervention model was built around existing public health infrastructure to support use of the self-assessment instrument and encourage environmental changes at the child care level, and this intervention model became the Nutrition and Physical Activity Self-Assessment for Child Care (NAP SACC) program. The NAP SACC program was designed for dissemination and has potential for implementation in many settings. Broad interest in NAP SACC has been expressed by a number of states and institutions, and many groups are using NAP SACC intervention and materials. The NAP SACC program shows promise as a useful approach to promoting healthy weight behaviors in child care settings.