This article makes two main propositions about the role of due diligence in international law, in response to recent interest in the topic. First, a legal requirement to exercise due diligence may be a component part of a primary rule of international law, but this can only be determined by referring back to the primary rule in question (eg what degree of fact-finding does treaty provision X require a State party to that treaty to undertake, either explicitly or implicitly, to act consistently with its terms?). In other words, there is no ‘general principle of due diligence’ in international law. Second, States undertake what could be characterised as ‘due diligence’ activity (eg by introducing policy guidance for their officials), some elements of which may be a result of a legal requirement and some of which may not (eg where done solely for policy reasons). Current practice of the United Kingdom and United States is used to illustrate the point. The lack of a distinction between the ‘legal’ and ‘non-legal’ elements of conduct in a given area gives States the flexibility to act without feeling unduly constrained by international law, and at the same time actually promotes compliance with international law and may assist in its development over time. In contrast, pushing for a ‘general principle of due diligence’ in international law is unnecessary, and risks having a chilling effect on this positive legal/policy ‘due diligence’ behaviour by States.