The notion of sovereignty plays an important part in various areas of law, such as constitutional law and international public law. Though the concept of sovereignty as applied in constitutional law differs from that used in international public law, there is no true consensus on the meaning of “sovereignty” within these respective fields, either. Is sovereignty about factual power, or only about legal equality? Do only democracies have sovereignty, because they have legitimacy, or is there no (necessary) connection between democracy, legitimacy and sovereignty? Has the European Union encroached upon the sovereignty of the Member States, or is transferring competences to the European Union an expression and exercise of the very sovereignty some claim is under attack? Is it about states, or is it about peoples having a right to self-determination, and if the latter, does this represent popular sovereignty or something else? In order to answer these and related questions, we need a clear grasp of what “sovereignty” means. This book provides an analytical and conceptual framework for “sovereignty” in the context of law. The book does not seek to describe how the term “sovereignty” is used in the different contexts and discourses in which it is employed, but rather distinguishes between two possible meanings of sovereignty that allow the reader to use the term with specificity and clarity. In this way, this book hopes to offer valuable analytical tools for politicians, constitutional and international lawyers (both practitioners and academics) and legal theorists that help them be clear about what they mean when they speak of “sovereignty.”