The Ambulatory Sentinel Practice Network (ASPN) was created to increase the knowledge of primary care. Building on the experiences of other national and regional primary care research networks, ASPN has evolved as a North American network including practices in 25 U.S. states and four Canadian provinces in 1987. This paper summarizes ASPN's growth and development since 1978, the involvement of the ASPN practices, and the mechanisms used in developing and managing studies.