PURPOSE One way to ensure quality of health care in the United States is through maintenance of certification (MOC). In this study, the authors explored whether participation in the internal medicine MOC program varies by physician-level characteristics, professional activities, and the size and location of the practice. They also sought to determine which component of MOC was incomplete for physicians who participated but did not complete the program. METHOD The authors used a theoretical realist approach to understand whether participation in the American Board of Internal Medicine MOC program varies according to physician and practice characteristics. The data came from a study sample that consisted of all physicians whose original certification was granted in internal medicine from 1990 through 1999; the study was conducted in 2013. Chi-square tests of independence and a multinomial logistic regression were conducted to determine which physician-level characteristics, professional activities, and practice characteristics were significantly associated with MOC participation. RESULTS Results showed that physicians who completed MOC tended to have higher certification exam scores; were younger; were U.S. medical graduates; practiced as subspecialists and in the Midwest; spent more time in patient care, teaching, or administration; worked in nonsolo practices; or were employed in counties with less than 20% of persons in poverty. CONCLUSIONS As certifying boards evaluate their programs, they need to continuously improve their features to assure the public that physicians maintaining certification are providing high-quality patient care.