At the beginning of its development, the science of international law was inextricably linked to the doctrine of natural law. The latter was seen as the basis of international law. The very problem of the foundations of international law became acute in the 19th century, when the prevailing legal positivism abandoned the idea of natural law. All proposed solutions were based on the idea of self-obligation of sovereign states. Some of them questioned the very existence of international law, while others required the introduction of explicit fictions. In an attempt to solve this problem, the pure theory of law developed by Hans Kelsen and his students proposed a theory of a hierarchical structure of international and domestic law. The relationship between the levels of the normative system is based on the empowering norms, which transfer the property of legal validity to the lower norms created on their basis. The concept of validity corresponds to the concept of efficacy of the norm. The interrelation of validity and efficacy of legal norms in international law differs significantly from their interrelation in domestic law; the study of this relationship in Kelsen’s theory was the main purpose of this study. The structure of international law according to Kelsen is a pyramid, the highest level of which is customary international law, based on the basic norm of international law that establishes the binding force of international custom. In this case, from the point of view of the pure theory of law, a special role in international law is played by the principle of effectiveness — recognition of the existing factual state of affairs as legitimate. The greater importance of this principle in international law is explained by the absence of a centralized system of coercion in the latter because decentralized legal order does not allow the application of organized sanctions in instances of violation of international legal norms.