Customary international law is one of the formal sources of international law and plays a pivotal role in the existence and functioning of the international legal system. Although for a rule of CIL to emerge a widespread, representative, constant and virtually uniform state practice is required, accompanied by the requisite opinio juris, that does not necessarily mean that CIL is a slow and archaic process, which has been overcome by extensive treaty-making. On the contrary, CIL remains a vital element in the corpus of international law that is open to refinement, clarification, development and evolution. This process does not happen only through the classical emergence and/or subsequent modification of the rule, but also and perhaps most importantly through the process of interpretation. This chapter demonstrates this by showing that CIL interpretation is neither problematic from a theoretical perspective, nor is it the only example of interpretation of unwritten rules. This is further reinforced by jurisprudence taken from both the domestic and the international legal system, although such interpretative exercises are not without their limits. What emerges from this analysis is that CIL interpretation, as Sur has beautifully noted, is of a negentropic nature that constantly nourishes and updates CIL.