A wave of populism is now sweeping across the advanced democracies in the northern hemisphere, overturning the conventional sense that since the mid-twentieth century populism has been primarily a phenomenon of countries with personalized, presidential political systems and radical inequalities, with Latin American countries providing prominent examples. Populism evidently poses a major challenge to prevailing political systems, placing pressure in particular on established patterns of partisanship and hence on political parties. These factors, along with increasing reliance on “directly democratic” decision-making mechanisms such as referenda, raise questions about some very basic aspects of the rule of law and other features of modern constitutionalism.