Aust’s chapter deals with the role of domestic courts in treaty interpretation. It analyses how domestic courts make use of the rules of interpretation set forth in the Vienna Convention on the Law of Treaties. These rules belong to the category of secondary rules of the international legal order. A diverging approach among domestic courts might undermine the consistency and coherence of international law. The chapter provides an overview of what the Convention sets out as ideal interpretative programme, how this question is related to notions of the international rule of law and how the process of interpretation is dealt with in domestic court decisions. It argues that adherence to the rules set forth in the Vienna Convention can strengthen the international rule of law.