Community health centers have evolved from fringe providers to mainstays of many local health care systems. Those designated as federally qualified health centers (FQHCs), in particular, have largely established themselves as key providers of comprehensive, efficient, high-quality primary care services to low-income people, especially Medicaid and uninsured patients. The Center for Studying Health System Change's (HSC's) site visits to 12 nationally representative metropolitan communities since 1996 document substantial growth in FQHC capacity, based on growing numbers of Medicaid enrollees and uninsured people, increased federal support, and improved managerial acumen. At the same time, FQHC development has varied considerably across communities because of several important factors, including local health system characteristics and financial and political support at federal, state and local levels. Some communities--Boston; Syracuse, N.Y.; Miami; and Seattle--have relatively extensive FQHC capacity for their Medicaid and uninsured populations, while other communities--Lansing, Mich.; northern New Jersey; Indianapolis; and Greenville, S.C.--fall in the middle. FQHC growth in Phoenix; Little Rock, Ark.; Cleveland; and Orange County, Calif.; has lagged in comparison. Today, FQHCs seem poised to play a key role in federal health care reform, including coverage expansions and the emphasis on primary care and medical homes.