In general, patients expect that the medical treatment they receive is provided by physicians who adhere to professional norms which are based on evidence. The existence of variations in medical practice challenges that general belief. In the assumption that treatment by physicians is based on theoretical knowledge and the medical condition of the patient, similar patients with similar conditions would receive the same treatment, irrespective of the physician, hospital or practice they attend. However, examples from literature show a different situation. In this book several hypotheses on the causes of medical practice variation were empirically tested. An approach was applied that emphasized social conditions that, by providing opportunities and constraints, influence physicians’ medical behavior. This approach predicts variation between physicians working in different work environments and similarities between physicians sharing the same work environment, such as a practice or a hospital. The importance of social organization and institutional mechanisms was confirmed in explaining medical practice variation. Data were used from two national data collections held in the Netherlands amongst general practitioners: the First and Second Dutch National Survey of General Practice (DNSGP1 and DNSGP2). These data were collected in 1987 and 2001 respectively. Furthermore, data were obtained from the New York Statewide Planning and Research Cooperative System (SPARCS). The 1999, 2000 and 2001 SPARCS-data were used. The analyses in this book were performed using multilevel analyses Three mechanisms that generate variation between and homogeneity within groups were tested: selection, gradual adaptation towards group norms and rapid adaptation to circumstances. The results pointed towards the importance of similarities based on shared circumstances. It also showed that the most common explanation of medical practice variations based on individual preferences was unsatisfactory. The implication is that medical practice ...