This paper offers a theory to explain the diffusion of new medical technologies into local practice. Based on several hundred interviews with community hospital physicians, it anchors technology decisions in the norms and relationships of local practice. Physician descriptions of their use of different types of assessment information provide insight into the way in which local consenses on appropriate practice are formed, guide behavior, and change. To understand new technology adoption, it is necessary to (a) differentiate "formed" (complete) and "dynamic" (still developing) technologies, and (b) appreciate the extent to which medical practice is locally organized. Concepts from organizational literature, then, become useful in explaining the penetration of these medical communities and the circumstances under which a new modality takes hold in them. Within the framework presented, previously puzzling findings regarding variations in local practice and the poor relationship between practice behavior and the published literature become understandable.