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In a pair of recent articles, Professors Jack Goldsmith and Eric Posner have used game theoretic principles to challenge the positivist account of customary international law. Their writings join other early attempts to apply game theory to the international law sources. This article has two purposes. The first is to evaluate game theory's potential for yielding greater insight into customary international law and international law more generally. The second is to respond to the conclusions about customary international law drawn by Professors Goldsmith and Posner. In Part I, Professor Chinen discusses the approach proposed by these two scholars. Traditionally, customary international law is understood as a general and consistent practice of states followed out of a sense of legal obligation. In Part II, he puts Professors Goldsmith and Posner's descriptive theory into some perspective by exploring whether the purported failure of customary international law to describe state behavior impacts the ability to use that law, first, as a tool for influencing state behavior, and second, as a source of rules of decision. In Part III, Professor Chinen argues that there is a sufficient amount of "true" cooperation among states to be construed as general and consistent state practices. In Part IV, he examines the concept of opinio juri. Finally, in Part V, Professor Chinen discusses directions for further research.