This book offers the first comprehensive legal study dedicated to the understanding of the Danish EU opt-outs. The impact of these is significant, falling as they do within Union citizenship, the euro, defense cooperation and the Area of Freedom, Security and Justice. Through a re-examination of the opt-outs individually, collectively and temporally, the book sheds light on their legal design and their interplay between international law, EU law and national law. This pioneering book takes a legal-doctrinal approach, which provides readers with a solid understanding of the opt-outs. Academics, judges and European Union civil servants will find this invaluable.