The rapid growth in investor–state dispute settlement has sparked a decades-long legitimacy crisis in the international investment regime. The upshot is that concerns over foreign investor success in arbitration proceedings, from levels of compensation to lack of arbitral diversity and independence, has prompted wide-ranging reform efforts and even calls to abolish this treaty-based system. In the context of this historical and contemporary debate, the authors describe the importance of assessing empirically the claims and counter-claims about the regime’s absence of legitimacy. The chapter begins with an overview of the different types of legitimacy (normative, sociological, legitimation) and discusses how and to what extent empirical research can contribute to assessing legitimacy claims. The different chapters of the book, which is structured largely according to legitimacy categories, are then introduced and the piece concludes with some reflections on the overall themes and way forward for empirical research.