Operating against anarchy rather than hierarchy, international constitutionalization is a highly contested but unintended political process. It is a by–product of international treaty making, which is intended to create fairly specialized rules within issue–specific domains. Approximating the evolutionary pathway, this process nonetheless generates a less unified international constitutional framework than in the national setting. An examination of international constitutional developments (along the five core constitutional elements) reveals that formalization of international rules through treaties has become standard in the post–1945 period. Sovereignty has developed into the fundamental international principle, extending the circle of members to include newly emerging states. Relations among states have deepened to enhance inter–state cooperation, and rights and duties have further reproduced inequalities among states. The chapter concludes by stressing the inadvertent constitutional consequences of international treaty making, which leads to the constitutionalization trap.