Does every woman who has a miscarriage require a dilation and curettage (D&C)? Does every patient with new-onset headache require a computed tomographic (CT) scan? Does every child with otitis media need a 10-day course of antibiotics? Does every woman with pelvic inflammatory disease (PID) require hospitalization? These are popular and appropriate questions to ask and investigate in 1999 but challenged conventional medical wisdom in the early to mid 1980s, when a group of practicing family physicians in the Ambulatory Sentinel Practice Network (ASPN; http://www.aspn.denver.co.us) began a series of descriptive studies of current practices for common clinical dilemmas they faced in everyday practice. Their early work suggested that the then-current National Institutes of Health Consensus Guidelines for use of CT scanning in new-onset headache were not being followed and that the gap between practice and the guidelines led to no measureable harm to patients1-3 and to a considerable reduction in health care costs.4 Subsequent studies further questioned the appropriateness of routine D&C after uncomplicated miscarriage,5 hospitalization of every woman with PID,6 and the routine use of a 10-day course of antibiotics for children with uncomplicated otitis media.7 In conducting these early studies, ASPN pioneered methods for low-cost, low-burden studies that collect high-quality data in busy primary care practice settings4 and laid the groundwork for studying the common phenomena of primary care. Future work will advance understanding of appropriate care for these and other clinical questions faced by primary care physicians in their daily work.