We propose to integrate a multi-site ethnographic study of treatment decisions made during inpatient surgical care episodes with information on the evolving structure of intra and inter-hospital physician networks derived from Medicare claims data. Linking qualitative and quantitative data in order to establish the individual, structural, and organizational mechanisms that drive network dynamics in the context of surgical care will make fundamental contributions to network and organizational theory while speaking directly to practical efforts to control health care costs. Dynamics of tie formation and updating underpin the evolution and effects of different hospital wide networks linking physicians to each other through the care process. Structural differences in care networks, which can be identified using Medicare payment data, have significant implications for health care quality and cost. At the same time, theoretically directed observational field work will offer new insights into the scope conditions of network effects and the role of caregivers (such as physician?s assistants and nurses) whose work is less easy to observe in large scale quantitative data.