This chapter considers the role of customary international law (CIL) in a world in which the treaty has become the predominant instrument of international legal cooperation. Far from rendering CIL irrelevant, the chapter argues that the increased use of more formal, institutionalized legal agreements to govern interstate relationships can actually increase the importance of CIL, both by increasing the credibility and clarity of customary rules and by relying on the interstitial nature of CIL to reduce the costs associated with contracting between states.

We employ a functionalist theory of CIL, in which the beliefs of states are the only relevant criteria for determining whether a rule of CIL exists. In other forms of international law such as treaties, an explicit agreement and its associated bargaining and ratification procedures serve to clarify states' beliefs and expectations. The lack of procedure involved in the creation of CIL thus creates a problem of clarity. Where customary rules are vague in terms of content or application, their compliance pull is necessarily reduced.

We argue that there are at least two ways states can employ “hard” legalization to increase the compliance pull of rules of CIL. First, the codification of custom can serve a signaling function. Because violations of treaties are generally more costly than violations of CIL in terms of both direct and reputational sanctions, codifying custom can send a costly signal to non-signatories about the legal beliefs of signatories. Using treaties as signaling devices thus allows signatories to increase the clarity of rules of CIL, and therefore the compliance pull of customary rules over all states, regardless of whether they accede to the treaty.

Second, tribunals can be used to clarify the content of rules of CIL, as well as to create information about violations, both of which increase the compliance pull of CIL. Significantly, domestic tribunals with jurisdiction over causes of action predicated on customary rules may be more important in this respect than international tribunals. Domestic tribunals are more likely to be able to enforce judgments based on CIL against their own governments, and because they are more numerous, they create many additional fora in which claims based on CIL can be heard.