The intense engagement of populists with constitutionalism—a phenomenon originally related to experiences in Latin America—is increasingly evident in some of the new European Union member states. But the populist phenomenon is clearly not confined to more recently established democracies. Populist constitutionalism stands for a number of distinctive tendencies in constitutional politics and practices which frequently are in tension with—and may even threaten—fundamental values, human rights, representative democracy, and the rule of law. The relation between populism and constitutionalism is, however, not necessarily one of anti-thesis, but rather manifests itself in distinctive ways, depending on specific contexts and variations. In this special issue, we argue that populist constitutionalism is best analyzed in a comparative, and historically and contextually attuned manner. The special issue wants to contribute to understandings of populist constitutionalism, which are both theoretically more robust and able to comparatively reflect on a diversity of “really existing” cases. The various contributions discuss central dimensions to the populist phenomenon. These pertain in particular to: (a) The varieties of populist engagement with constitutionalism; (b) a deeper understanding of the populist mindset; (c) the position-taking and reaction of constitutional scholars to populism; (d) the complex relation and overlap of populism with illiberalism and authoritarianism; and (e) the central nature of constituent power in populist projects.