As scholars in the Global Administrative Law project have recognized, doctrines familiar from domestic administrative systems are beginning to appear, in nascent forms, in some areas of international law. This article makes a first attempt to examine the appearance of one such doctrine, the duty to give reasons for administrative decisions, in international case-law. The existence of and rationales for this duty have been contentious in many domestic jurisdictions. The article thus considers the extent to which these debates have been replicated amongst adjudicators at the international level. The focus is on cases in the areas of WTO law, investment law and human rights law. It is found that the case law is not yet extensive, and (perhaps as a result) that no coherent picture emerges. In contrast to domestic systems, the areas examined in international law demonstrate some agreement on the desirability of the duty. However, different international adjudicators have recognized different rationales for the duty, with only limited consensus even within each area of international law studied.