This chapter focuses on some of the Third World Approaches to International Law (TWAIL)s criticism of investment law and the CIL rules contained therein, and proposes an engagement with this criticism at the stage of interpretation. Based on an examination of the interpretation of the customary minimum standard of treatment, it argues that interpretation plays two roles in relation to CIL – a constructive and an evolutive role. In light of these two roles, interpretative arguments may be deployed strategically so as to flag problematic rationales in the rules and argue for their re-interpretation. In particular, I propose three interpretive strategies which may achieve this goal, which are differentially suited to the different actors who may wish to deploy them. These strategies are limited by the actors who deploy them, the forum where they are deployed, as well as the rules of procedure in which the dispute takes place. Nevertheless, they present an opportunity for a constructive TWAIL engagement with international investment law which does not summarily dismiss the existing system.

This chapter examines the contribution that domestic courts may have in the development of rules or guidelines for the interpretation of customary international law (CIL). Through an examination of national cases where courts interpret both domestic and international custom, the chapter traces methodologies of interpretation and motivations behind them. The chapter then asks two questions: how can we learn from domestic interpretive practices? and why should we learn from them? In answer to the first question, the chapter argues that domestic courts can contribute to the development of rules for CIL interpretation beyond the role assigned to them in the framework of sources, and also in informal ways. In answer to the second question, the chapter argues that looking to domestic interpretive approaches for custom may help us fill the ‘gap’ currently existing on this issue in international law, that domestic courts offer a wealth of cases from which we can draw, and that by learning from domestic interpretive practices international law can provide domestic judges with various familiar tools for their further engagement with CIL in the future.