The medical home is a potentially transformative strategy to address issues of access, quality, and efficiency in the delivery of health care in the United States. While numerous organizations support a physician-driven definition, it is by no means the universally accepted definition. Several professional groups, payers, and researchers have offered differing, or nuanced, definitions of medical homes. This lack of consensus has contributed to uncertainty among providers about the medical home. We conducted a systematic review of the literature on the medical home and identified 29 professional, government, and academic sources offering definitions. While consensus appears to exist around a core of selected features, the medical home means different things to different people. The variation in definitions can be partly explained by the obligation of organizations to their members and whether the focus is on the patient or provider. Differences in definitions have implications at both the policy and practice levels.