This chapter introduces two new GIS-supported methods of measuring accessibility to primary healthcare. The improved floating catchment method defines the service area of physicians by a threshold travel time while accounting for the availability of physicians. The gravity-based accessibility method considers two factors: travel times from service providers (a nearby supply is more accessible than a remote one) and competition intensity by residents for such a service (measured by gravity-based potential). The methods are applied to examining accessibility to primary care in the northern Illinois region in 1990 and 2000. The GIS-based methods may be used to help the U.S. Department of Health and Human Services and state health departments define health professional shortage areas.