Interpretation of Customary International Law in International Courts

Customary international law (CIL) is one of the sources of law that
regulates the relationships between States. As with treaty rules, that is,
rules contained in international conventions, in order to be applied to
particular cases, customary international law needs to be interpreted.
This thesis explores the topic of the interpretation of international custom
and, in particular, the methods that international courts and tribunals
and quasi-judicial bodies have used to interpret customary rules in their
practice. Its aim was to bring to light in a systematized manner the
different methods of interpretation that can be observed in the case law
of international courts and quasi-judicial bodies.
This study not only disproves the fact that customary rules cannot be
interpreted, but also shows that in order to interpret rules of customary
international law, international courts, quasi-judicial bodies and
international judges, in their separate or dissenting opinions, rely on
methods of interpretation that are either identical or similar to those
contained 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 set
of acceptable methods or rules of CIL interpretation that will guide judges
in crafting their arguments and, as a result, will increase the transparency
and persuasiveness of judicial reasoning.