Solidarity amongst EU member states is increasingly being called for - especially in times of crisis. 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. In the theoretical section, Barteinstein develops a concept of solidarity, which identifies the conditional factors for the institutionalisation of solidarity. In the empirical section, 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 has begun to emerge in the EU.