This book analyses how the Appellate Body uses particular principles of general international law in interpreting the WTO covered agreements. It deals equally with general international law and WTO law. The aim is to explain how the Appellate Body interprets and applies customaryinternational law on treaty interpretation in dealing with the WTO covered agreements. The main concern is to analyse the judicial reasoning and ways of justifying judicial decision-making. In particular, it answers the question of how the Appellate Body explains its reading of WTO treatylanguage.It is argued that the Appellate Body has interpreted the WTO covered agreements in a contextual and effective manner, an approach that corresponds with general international law. The character of the WTO covered agreements has, nevertheless, confronted the Appellate Body with some questions ofinterpretation that were until recently unexplored or neglected by other courts and tribunals. In that sense, the Appellate Body has contributed to the development of general international law on treaty interpretation, or at least to its practice.WTO law is primarily treaty law, but increasingly soft law and broader themes and values from other disciplines, such as governance, variable geometry and legitimacy, are introduced and discussed. Customary international law - with the exception of the principles of treaty interpretation - andgeneral principles of law are often seen as excluded entirely. An ancillary theme of this proposed monograph is the extent to which customary international law and general principles of law have penetrated WTO law through the technique of treaty interpretation.