Community definition is an important aspect of community health work in general and community-oriented primary care (COPC) in particular. Yet, community definitions are often nonspecific, relying on geopolitical boundaries or local presumptions about patient populations. Such definitions are an impediment to the precise application of sociodemographic or health status data to community health problems or to targeted community organizing. This paper describes a technique called "geographic retrofitting" that has proved useful in establishing a rigorous definition of a practice's community based on current patterns of health care by the community. It also demonstrates how this approach, used in conjunction with geographic information systems software, facilitates more-powerful capabilities in community characterization and intervention.