The uncertain character of customary international law (CIL) has been discussed time and time again among academics and practitioners. To most of them, the uncertain character of CIL entails a twofold defect: first, uncertainty is perceived with respect to the identification of the rule, since we may not know whether there is a valid legal rule; and second, uncertainty is perceived with respect to the content of the rule, since we may not know the precise meaning of the rule. Yet, what seems to be missing from the entire discussion is the mechanism by which CIL functions. Although a number of theories have been formulated, there are no persuasive answers that would explain when and how changes in CIL occur. In other words, the dynamics of CIL, as the driving force behind its evolution remain essentially unexplored. Providing answers to these questions requires an in-depth understanding not only of the structure of CIL but also of the processes that occur in and out of CIL during its operation as a social system. This chapter uses complexity science to describe the functioning of CIL and explore CIL’s construction as a social system.