Populism and Civil Society: The Challenge to Constitutional Democracy is a theoretical work that draws on extensive secondary literature as well as comparative analysis of cases. The aim is to assess the significance of what is now a global phenomenon—the populist challenge to constitutional democracy. After defining populism using the methods of immanent criticism and ideal typic construction, the book proceeds to examine the challenge in terms of its four main organizational forms: movement mobilization, political party, government, and regime. It considers the important questions: “why populism?” and “why now?” Without presupposing the authoritarian logic of the phenomenon in the definition, the book seeks to demonstrate this logic through the reconstruction of the main elements used by advocates to identify populism. It shows that authoritarian logic is not fully realized in every empirical form of populism and considers why this is so for many movement and party forms, and even populists “in” government vs. populism as “the” government. We identify the historical examples of the latter as “hybrid” regimes blending authoritarian elements and residual democratic forms. The book then proceeds to consider the uneasy relationship of populism to constitutionalism and presents populism as a form of abusive or instrumental “constitutionalism” often relying on the alleged permanence of the quasi-revolutionary constituent power. It concludes by outlining a non- and anti-populist project of democratization and social justice, distinguishing between the “popular” and the “populist,” and outlining a program based on the plurality of democracies and the rescue of some of left populism’s “host ideologies.”