With the treaty of Lisbon, the solidarity principle gained influence in EU primary law and serves as a structural principal of the European Union. The book discusses the theoretical concepts of solidarity and applies them to the context of multilevel systems. The contributions focus on different policy fields, including economic policy, immigration and asylum policy, energy policy, structural funds, Common Foreign and Security Policy etc. The authors pursue different research questions and theoretical approaches as they represent different disciplines, such as economic science, European law and political science.