In many, if not most cases, the ECtHR relies on its own precedents in interpreting the Convention, but some cases may present the Court with new and difficult questions of interpretation. To answer these, the Court relies on a number of specific principles and methods of interpretation. The Court is guided in its work by several core principles, such as effectiveness, evolutive interpretation and autonomous interpretatio, but these are still relatively abstract in nature and they may not suffice to answer a concrete question of interpretation. For that reason, the Court often also relies on the methods of interpretation as described in the provisions of the Vienna Convention on the Law of Treaties, such as textual interpretation, interpretation in light of the travaux préparatoires and internally harmonising interpretation. This chapter discusses the Court’s use of these three methods. In addition to these methods, the Court has opted for a particular refinement of one of the Vienna Convention’s methods, which is consensus or common ground interpretation. This method and the various sources for the Court’s finding of a consensus are also discussed in this chapter.

Does judicial interpretation of IIAs produce epistemic ‘authority’? Interpretation can mean both the process of finding out what texts mean and guidance to the concretisation of abstract general norms in individual instances. In the second sense, interpretation is subconsciously used to narrow the freedom of deciders; a range of interpretative tools is used to generate a quasi-formal unity of meaning across IIAs. Systemic integration is the most used and most potent tool. It enjoins us to assume that meanings are identical, but this is baseless: taking into account external rules could just as easily be the basis for a divergence. However, customary law is less certain and precise than assumed. From the perspective of peer-accepted reasoning-before-decision, the interpreter takes meanings, not external rules, into account. Arbitral tribunals in fact interpret IIAs in light not of a customary norm but of other investment tribunals’ understanding of the meaning of other treaty norms. On that perspective, there is no distinction between interpretative tools. Interpretation cannot unify investment law because it does not change the law, only certain brute facts.