Traditional justifications for the authority of international courts are based on outmoded assumptions of the role and impact of international courts. State consent and procedural fairness to litigants are insufficient to ground the normative legitimacy of institutions which may adjudicate the international rights and duties of non-litigants, deeply affect the interests of non-litigating stakeholders and shape the law prospectively. These realities mandate a new approach to the normative legitimacy of international courts. This article presents three alternative or additional approaches for justifying the authority of international courts. First, legitimacy requires a re-imagining of procedural fairness to include those whose international rights and duties are being adjudicated by international courts. Further, democratic theory can help to justify the authority of international courts so long as stakeholders are given the opportunity to participate in the formulation of policies that affect them. Second, international courts must adhere to certain universal standards of justice. They cannot facilitate the violation of a set of core norms, including prohibitions against torture, slavery, racial discrimination and genocide, and still retain their legitimacy. Third, the extent to which an international court implements the objectives it was created for may also affect its legitimacy. The article’s objective is to spark discussion about the standards by which these increasingly important institutions should be judged.