Introduction

The number of books written on treaty interpretation in recent years speaks to the importance of the topic. It has been said that ‘[o]f all the issues raised by the Vienna Convention on the Law of Treaties, there can be few which combine the theoretical interest and practical importance to the same degree as the question of treaty interpretation’. The reason is that ‘[m]ost disputes submitted to international adjudication involve some problem of treaty interpretation’, and therefore ‘[j]ust as the interpretation of legislation is the constant concern of any government lawyer, treaty interpretation forms a significant part of the day-to-day work of a foreign ministry legal adviser’. Article 3.2 of the DSU states that the WTO dispute settlement system serves to clarify the existing provisions of the WTO agreements ‘in accordance with customary rules of interpretation of public international law’. WTO panels and the Appellate Body have confirmed the customary international law status of Articles 31, 32 and 33 of the Vienna Convention, and there is now a very substantial body of WTO jurisprudence regarding the various elements of these provisions. As far back as 1999, one WTO panel observed that ‘[i]n recent years, the jurisprudence of the Appellate Body and WTO panels has become one of the richest sources from which to receive guidance on their application’. Statements by WTO adjudicators relating to Articles 31, 32 and 33 of the Vienna Convention are of wider relevance because those provisions reflect the general principles of interpretation that apply to any treaty, irrespective of its subject-matter. As the Appellate Body explained in US – Hot-Rolled Steel: