The International Law Commission held that that the conduct of non-State actors ‘is not practice that contributes to the formation, or expression, of rules of customary international law’ for the sake of the identification of customary international law. However, the present chapter argues that the activities of de facto regimes can be considered as properly speaking ‘practice’ that contributes to the formation, or expression, of rules of customary international law under two scenarios. Firstly, the custom-creating or custom-strengthening conduct of the de facto regimes might be attributed to a State, usually to an occupying power or a State supporting in various means the entity by exercising effective control or at least decisive influence over it. Secondly, de lege ferenda, even without attribution, the conduct of de facto regimes should be taken into account as strengthening the practice forming customary international law at least in subject matters governing the rights and obligations of the de facto regimes itself. This chapter argues that in both scenarios the de facto regimes’ law-making capacity and conduct should be taken into account either as relevant sources for identifying the ‘practice’ of States or the conduct of international organisations, or as relevant ‘practice’ as such.