This chapter delves into the concept of legitimacy and introduces the readers to key debates on regulatory legitimacy. The concept of legitimacy has been extensively studied by scholars from various academic disciplines, including political theory, legal theory, political science, sociology and management studies. The resulting body of scholarship has, however, tended to remain in disciplinary siloes, making the study of legitimacy difficult to navigate. Chapter 11 offers first an exploration of different legitimacy claims that justify why individuals recognize an authority and its rules as legitimate. The chapter then moves to regulatory legitimacy.