In the last decade, populist leaders and parties have been increasingly successful around the world. In some cases, these movements have even been able to gain public power. This presents a challenge for constitutionalism and constitutional law. For some, populism is by definition irreconcilable with constitutionalism. At the same time, populist in powers have often used or borrowed constitutional law concepts and more generally rely on constitutional theory to justify their actions. What is exactly the relationship between constitutionalism and populism? Is populism always a threat to constitutionalism? How to understand the new wave of populism, and how can constitutions react to populism?