Our longitudinal study examines the changing roles of health care professionals (physicians, nurses, medical assistants, practice managers, and secretaries) before and after an EHR implementation in a large, multi-location group practice. We take a social informatics perspective and focus on the changing social identities of health care professionals as they adapt to their EHR-enabled roles. A year after go-live, a few professionals were still in reactive mode, trying to cope with the new system, but many others were actively shaping the technology and their roles in a variety of ways. A few went beyond shaping to find ways to provide additional value to themselves and to patients in ways that became possible only because of the EHR. In this paper, we explore these responses to the EHR as a basis for building theory about the potential for EHR systems to improve health care delivery, and the mechanisms by which that potential is realized.