Book excerpt: The rules of treaty interpretation codified in the 'Vienna Convention on the Law of Treaties' now apply to virtually all treaties, in an international context as well as within national legal systems, where treaties have an impact on a large and growing range of matters. The rules of treaty interpretation differ somewhat from typical rules for interpreting legal instruments and legislation within national legal systems. Lawyers, administrators, diplomats, and officials at international organisations are increasingly likely to encounter issues of treaty interpretation which require not only knowledge of the relevant rules of interpretation, but also how these rules have been, and are to be, applied in practice. Since the codified rules of treaty interpretation came into decree, there is a considerable body of case-law on their application. This case-law, combined with the history and analysis of the rules of treaty interpretation, provides a basis for understanding this most important task in the application of treaties internationally and within national systems of law. Any lawyer who ever has to consider international matters, and increasingly any lawyer whose work involves domestic legislation with any international connection, is at risk nowadays of encountering a treaty provision which requires interpretation, whether the treaty provision is explicitly in issue or is the source of the relevant domestic legislation. This fully updated new edition features case law from a broader range of jurisdictions, and an account of the work of the International Law Commission in its relation to interpretative declarations. This book provides a guide to interpreting treaties properly in accordance with the modern rules.