This article introduces the special issue of Comparative European Politics on Tolerant and Intolerant Responses to Populist Parties: Who does what, when and why? Populist parties have become more powerful political actors in Europe’s multilevel system of governance. Yet we lack concepts and comparative empirical studies charting how those who disagree with populist parties oppose them and theoretical accounts of the normative implications of this opposition. Discussing how the articles in this special issue address these shortcomings, this introductory article argues that while responses to populist parties is a neglected field in the study of populism, the ambiguity of populist orientations to liberal democracy sits uneasily with traditional paradigms for studying democracy’s challengers. It then outlines a new typology, developed with reference to case studies on Germany, Denmark, Italy and Poland, which aims to present a new conceptual language for understanding contemporary opposition to poulist parties. The article also explores the conceptual and institutional limits of political and legal (in)tolerance, a concept which provide a bridge between political science and political theory traditions encompassed in the special issue.