Especially in times of crisis, solidarity between EU member states is increasingly called for. Nevertheless, the concept of solidarity remains unclear. In her new book ‘The Concept of Solidarity - Energy Policy in the European Union', IFSH researcher Aline Bartenstein contributes to the clarification and analysis of this fuzzy concept. To this end, she develops a concept of solidarity in the theoretical part, which identifies the conditional factors for the institutionalisation of solidarity. In the empirical part, she applies the concept to the integration processes in the natural gas sector between 2002 and 2019. Her analysis shows that an institutionalised principle of solidarity is in the making in the EU.