How the WTO deals with regional trade agreements (RTAs) is conceptually and practically one of the most important questions in international trade law. This book clarifies that relationship focussing on one form of regional integration – customs unions – and one form of trade measures – anti-dumping measures. This book answers the question how anti-dumping measures and legislation change if a state is in a customs union as well. In doing so, this book provides a new reasoning why anti-dumping measures are modified in customs unions, as well as a comprehensive overview of how this has happened, a legal analysis on the legality of these changes, and an answer to the question how the different institutional settings have impacted questions of responsibility and attribution. Going beyond this, this book also considers the specific problems that arise in cases of economic integration and disintegration, and finally, the impact forming a customs union has on third parties that may imposeanti-dumping measures on states that are members of a customs union.