Bridging the gap between the disciplines of law and politics, this book presents a comprehensive review and analysis of the participation of national higher courts with powers of constitutional review throughout the ratification processes of European Union treaties. Unlike most studies of judicial behaviour, which focus on decisions on the merits, this book analyses interpretative decisions in the rulings. By focusing on the rulings of these courts and the political environment in which these rulings were issued, the author demonstrates that powers of constitutional review have been essential to EU Treaty reform.