Against the backdrop of normative arguments on the legitimacy of the international judiciary, this paper investigates empirically observable strategies of legitimation and de-legitimation of international courts and tribunals. Based on evidence gathered primarily from three institutional settings—the International Court of Justice, the European Court of Human Rights, and ad hoc investor-state arbitration—it yields, in a first part, an inductively generated taxonomy of strategies of legitimation and de-legitimation that have been employed by the courts under consideration and by their users, including strategies that implicate the democratic legitimacy of judicial review beyond the state, separation of powers issues, and the ever-fine line between judicial interpretation and judicial law-making. In a second part, I assess the effects that these strategies have had on the courts’ judicial authority in terms of acceptance of their jurisdiction and compliance with their decisions, and discuss the methodological challenges of conducting such “legitimacy impact assessments.”