The changes all physicians make in their treatment of patients constitute a neglected but key issue in the operation of the health care system. Identification of a model of this crucial change process was the purpose of this study. Interviews with a random sample of 66 physicians representing five specialties produced data on 182 changes. The fundamental stages in the change process were priming (coming to feel dissatisfaction with some aspect of practice behavior), focusing (learning of alternative practice behavior), and follow-up (obtaining further information or advice regarding the possible change). Any of a variety of information sources may focus a change, but follow-up is overridingly dependent on colleague communication (representing local professional opinion) and journals (representing an authoritative professional perspective). This change process model provides a basis for orienting continuing medical education activities to the achievement of behavioral outcomes.