In both national civil and common law systems the principle of reasonableness is known as a standard of judicial review for administrative acts. The present paper examines whether also at the European and international level a principle of reasonableness is used as a standard against which (quasi-) judicial bodies scrutinize administrative acts. It is argued that the common understanding exists that reasonableness serves as an important guiding principle in order to find agreement between the views of a multitude of actors, whether formally participating or not, in global arenas. Based on an analysis of cases before EU courts, the WTO Dispute Settlement Body, the ILO Administrative Tribunal and the UN Dispute Tribunal, it is submitted that although current European and international legal practice does not offer a single formula or test, the open-endedness of reasonableness makes it fit for use as a standard of judicial review at different layers of governance.