The interaction between domestic law and international law is a topic of perennial interest for international lawyers. Domestic law has long been recognised as a source of international law, an inspiration for legal developments or the benchmark against which a legal system is to be assessed. Often, it is simply treated as mere fact, indicative of the legality of a state’s actions. Academic commentary normally re-traces these well-trodden paths, leaving one with the impression that the interaction between domestic and international law has been thoroughly mapped, and is unworthy of further enquiry. However, a different – and surprisingly pervasive – nexus between the two spheres has been largely overlooked: the use of domestic law in the interpretation of international law. The present book aims to fill that gap in the literature.