Covering key episodes up to the attack on the US Capitol in January 2021, the fourth edition of this classic study emphasizes the symbiotic relations between social movements and parties by focusing attention on the growing role of populism in Europe, Latin America, and the US; analyzes the role of social media as a mobilizing and aggregating force for social movements; highlights the relations between structural changes in the economy and new forms of contention; draws on new material on movements in the Global South and the relations between movements and democracy.