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. Academic commentary normally re-traces these well-trodden paths, leaving one with the impression that the interaction between domestic and international law 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. This book examines the practice of five international courts and tribunals to demonstrate that domestic law is invoked to interpret international law, often outside the framework of Articles 31 to 33 of the Vienna Convention on the Law of Treaties. It assesses the appropriateness of such recourse to domestic law as well as situating the practice within broader debates regarding interpretation and the interaction between domestic and international legal systems.