Interpretation of Customary International Law in International Courts

Abstract

Customary international law (CIL) is one of the sources of law thatregulates the relationships between States. As with treaty rules, that is,rules contained in international conventions, in order to be applied toparticular cases, customary international law needs to be interpreted.This thesis explores the topic of the interpretation of international customand, in particular, the methods that international courts and tribunalsand quasi-judicial bodies have used to interpret customary rules in theirpractice. Its aim was to bring to light in a systematized manner thedifferent methods of interpretation that can be observed in the case lawof international courts and quasi-judicial bodies.This study not only disproves the fact that customary rules cannot beinterpreted, but also shows that in order to interpret rules of customaryinternational law, international courts, quasi-judicial bodies andinternational judges, in their separate or dissenting opinions, rely onmethods of interpretation that are either identical or similar to thosecontained in Article 31 of the Vienna Convention on the Law of Treaties.My hope is that it will contribute, in the long run, to the creation of a setof acceptable methods or rules of CIL interpretation that will guide judgesin crafting their arguments and, as a result, will increase the transparencyand persuasiveness of judicial reasoning