Political scientists use the concept of legitimacy to assess the rightfulness of political rule. Their research can approach legitimacy from two perspectives: When taking a normative approach, political scientists develop and justify their own evaluation of the rightfulness of political arrangements. When taking an empirical approach, they study how other people—such as political elites or citizens—evaluate the rightfulness of political rule. Both approaches have been used in research on the European Union. Scholarly discussions that approach the EU’s legitimacy from a normative perspective revolve around the question of which standards of rightfulness are appropriate for the EU. These depend largely on how the EU polity is conceptualized: as a technocratic regulatory agency, an intergovernmental organization, a federation, a demoi-cracy, or a system of multilevel governance. Since the EU is hybrid polity that possesses elements of each of these models, and is therefore difficult to classify, no consensus has emerged in this debate. Scholarship that approaches the EU’s legitimacy as an empirical phenomenon examines political attitudes and discourses in European society, asking whether, and why, societal actors treat the EU as legitimate. A diverse set of research methods—including public opinion surveys, content analysis of different kinds of texts, and qualitative interviews with citizens—have been applied to shed light on this question. While this research has not provided clear evidence of a “legitimacy crisis” of the EU, it does show that many Europeans relate to the EU with a sense of diffuse unease and skepticism, in part because they find it opaque and difficult to understand.