Interactions between state, international, transnational and intra-state law involve overlapping, and sometimes conflicting, claims to legitimate authority. These have led scholars to new theoretical explanations of sovereignty, constitutionalism, and legality, but there has been no close attention to authority itself. This book asks whether, and under what conditions, there can be multiple legitimate authorities with overlapping or conflicting domains. Can legitimate authority be shared between state, supra-state and non-state actors, and if so, how should they relate to one another? Roughan argues that understanding authority in contemporary pluralist circumstances requires a new conception of relative authority, and a new theory of its legitimacy. The theory of relative authority treats the interdependence of authorities, and the relationships in which they are engaged, as critical to any assessment of their legitimacy. It offers a tool for evaluating inter-authority relationships prevalent in international, transnational, state and non-state constitutional practice, while suggesting significant revisions to the idea that law, in general or even by necessity, claims to have legitimate authority.