This dissertation investigates authentic interpretation of treaties with dual aims: to understand unprecedented challenges against the authority of authentic interpretation, and to reflect on their consequences for international lawmaking. Having surveyed a variety of issue-areas, the dissertation argues that challenges against the weight of authentic interpretation typically arise when the rights of third-party beneficiaries to be adjudicated by international adjudicatory bodies, such as human rights courts and investor-state arbitration, will be adversely affected by that interpretation. Such challenges are often justified by a consistent pattern of legal arguments and typically shaped by an epistemic framework which may be called the ‘transnational paradigm’, as opposed to the ‘inter-state paradigm’ which dominated the discipline of international law for much of the twentieth century. The contestability of authentic interpretation has culminated in changes in authority relations in international lawmaking and erosion of state control over the process of treaty interpretation. Some international courts and tribunals, by contrast, have achieved the status of supreme treaty interpreters.