This volume approaches the current crisis of solidarity in the European Union from a multidisciplinary perspective. The contributions explore the concept of solidarity, its role in the European integration process, and analyze the risks entailed by a lack of solidarity. Experts from various academic fields, such as political science, law, sociology, and philosophy, shed new light on contemporary challenges such as the migrant and refugee crisis, the Eurozone crisis, nationalist and separatist movements, and Brexit. Finally, they also discuss different solutions for the most pressing problems in EU politics. The book has two main aims: Firstly, to show that solidarity is a key element in solving the EU's contemporary problems; and secondly, to reveal how the crisis of solidarity has become a crucial test for the integration project, as the nature of the crisis goes beyond the well-known shortcomings in the EU's structure and problem-solving capacities.