In this article the authors consider how to interpret the multilateral provisions in relation to tax administration such as the Common Reporting Standard (CRS), Base Erosion and Profit Shifting (BEPS) and the Mandatory Disclosure Regime, the pace of adoption of which continues to accelerate. They discuss whether the pre-existing framework provided by the Vienna Convention on The Law of Treaties is either adequate or appropriate in its application to the interpretation of such multi-later instruments. In light of this analysis the authors consider what principles of interpretation are found in case law and whether the principles ennunciated in Fowler v. Revenue and Customs Commissioners  1 All ER 97, could be used to remedy any deficiency in the Vienna Rules. This includes an examination of the dangers of the use of commentary in the interpretation of multi-later instrument and the importance of principles that allow the interpretative tradition of the relevant jurisdiction to override commentary. Having examined these difficulties and the inadequacies in the modern interpretive code, they propose methods of approach to ease the interpretative issues created by multi-lateral instruments, including the possibility of an internal code of interpretation for such multi-lateral instruments.