This chapter discusses the interpretation of the Energy Charter Treaty. The ECT is a multilateral treaty during the negotiation of which approximately fifty States participated, albeit to varying degrees. It goes without saying that in such a setting, there are many competing interests to take into account, often resulting in ambiguous treaty provisions. Indeed, almost every dispute based on the ECT involves issues of treaty interpretation. The law applicable to the interpretation of treaties is international law, unless the parties to the treaty in question have agreed otherwise. For all practical purposes, the most important international document when it comes to treaty interpretation is the 1969 Vienna Convention on the Law of Treaties. Article 2(1)(a) of the Vienna Convention defines ‘treaty’ as ‘an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation’. There is little doubt that the ECT is covered by this definition. Article 26(6) of the ECT provides that disputes under it are to be resolved on the basis of its provisions and ‘applicable rules and principles of international law’. The chapter then considers Articles 31—3 of the Vienna Convention, which deal with the interpretation of treaties.