This book explores the question of under what conditions rule is recognized as legitimate. It provides an introduction to various approaches to legitimacy theory and examines the evolution of the understanding of legitimacy over the centuries. Among other things, it describes legitimacy concepts according to Jean Bodin, Thomas Hobbes, John Locke, and Jean-Jacques Rousseau. The development of the democratic concept of legitimacy, which is based on the idea of popular sovereignty, is clearly traced, and the discussion surrounding the legitimacy deficit of the European Union is also addressed. The presentation of the criticism of the concept of legitimacy completes this successful treatise on the legality of rule.