This book adopts an innovative conceptualization and analytical framework to the study of anti-system parties, and represents the first monograph ever published on the topic. It features empirical research using original data and combining large-N QCA analyses with a wide range of in-depth case studies from 18 Western European countries. The book adopts a party-centric approach to the study of anti-system formations by focusing on the major turning points faced by such actors after their initial success: long-term electoral sustainability, the different modalities of integration at the systemic level and the electoral impact of transition to government. The author examines in particular the interplay between crucial elements of the internal supply-side of anti-system parties such as their organizational and ideological features, and the political opportunity structure. Anti-System Parties is a major contribution to the literature on populism, anti-establishment parties and comparative political parties.