Abstract: In two earlier articles, the tools of game theory were used to sketch a positive theoretical account of customary international law ("CIL"). This theory rejected as question-begging the usual explanations of CIL based on legality, morality, opinio juris, and related concepts. It was argued instead that CIL emerges from nations' pursuit of self-interested policies on the international stage. This approach helps explain many overlooked features of CIL, including how CIL originates and changes, why the content of CIL tracks the interest of powerful nations, and why nations change their views of CIL when their interests change. Finally, the practices associated with four supposedly well-settled rules of CIL were examined, and it was concluded that this theory better explained these practices than competing theories.