A variety of profiling models and tools is utilized by payers, providers, and regulators to evaluate physician work, performance, and resource utilization. In physician profiling, the provider's pattern of practice is expressed as a rate of service or outcome. The article by Tucker, Weiner, Honigfeld, and Parton (this issue) compares the practice-based norms of primary care physicians by adjusting for case mix using ambulatory care groups (ACGs), a population-based classification method. Once the case mix is adjusted, the actual use of resources, as measured by overall charges, is compared with the expected value of resource use. In the Center for Research in Ambulatory Health Care Administration (CRAHCA) Physician Profiling Project, funded by The Robert Wood Johnson Foundation, physicians learn which services other physicians in their specialties perform. Physicians are able to compare their profiles with state and national level medians. The profiling project is one of the first demonstration projects In the field to profile ambulatory care practice patterns and collect patient demographics. An aspect of the project is to test the ACG classification system to data selected from 130 nonacademic practices representing over 5,000 physicians.