Nearly all health systems and professionals espousean ideal of keeping to medical practitioners are purported to have special obligations to their patients. These include obligations to prioritize their patients over others in their professional actions. They must do so even when this requires foregoing the opportunity to produce a vastly more just distribution of healthcare. Yet, the foundations of these obligations are unclear at best. Some believe that these obligations arise from the aims of medicine or some (social) contract, but both types of foundations rest on spurious metaphysical and epistemic claims. Moreover, the claims for such goods, based in justice, of the exceptionally needed are particularly stark. Hence, if we cannot find better foundations for this ideal, there is grave risk that we do great harm and grave injustice in complying with it. This dissertation criticizes standard views of these foundations and then offers an alternative according to which the legitimate prescriptions of legitimate medical practices as well as those of legitimate governments. The concept of legitimacy is already widely invoked in medical ethics—particularly to justify and clarify the demands of resource allocation, such as those that must cohere with the special obligations mentioned above. Unfortunately, typical views of legitimacy, to which one might appeal, cannot capture the right extension of verdicts about resource allocation in critical cases.  In light of these failings, this dissertation turns to the foundations of political philosophy to develop a new theory of legitimacy and authority of government and then extend that theory to explain the legitimacy and authority of medical practice. According to this theory, entities have authority over their subject if, only if, and because the subjects would better fulfill their associative duties to each other. It then uses this theory to illuminate important claims in the general theory of partiality. This paves the way to developing a general framework for understanding legitimate medical practice from this theory, the implications of which the dissertation illustrates by expounding both on the theory of special obligations of medical professionals and on the theory of healthcare resource allocation in regard to claims of medical necessity.