When faced with the inevitable task of interpreting customary international law (CIL), what should a court do and what should it consider? Should it engage in an ‘inductive’ process of sifting through available evidence of state practice and opinio juris or a deductive process designed to reason logically from principles embedded in the rule? Should the court invoke something like the rules of treaty interpretation with their focus on good faith, ordinary meaning, context, and object and purpose? International law doctrine falls short here. Figuring out how to interpret and apply custom requires a theory of custom, a focus on the normative stories we tell. This reveals that there is not just one story explaining why custom should be a source of law, but multiple stories. What we call ‘custom’ may represent or draw from at least three different sources of law: Negotiated Law, Legislated Law and Adjudicated Law. This chapter aims to show that the non-treaty rules derived from each draw on different sources of legitimacy, operate according to different logics, dictate different methods of interpretation, and favour different methods for resolving disputes.