During the past 20 years, the feasibility of practice-based research in networks has been established in the United States. The initial work of these networks has revealed the need for a better understanding of family practice and the rest of primary care in order to address the challenges facing our health care system. This paper explores the nature, potential, and limitations of practice-based research networks based on the results of a dozen studies conducted by the Ambulatory Sentinel Practice Network (ASPN).