Solidarity is a key concept in the European Union. However, the concept of solidarity lacks systematic theoretical examination to enable a sufficient understanding of its contextual meaning and to provide an operationalisable benchmark for analysis. To address this research gap, I propose an analytical framework for solidarity in the European Union that features four necessary conditions: particularity, instrumentality, reciprocity, and responsibility. I develop the framework through a transdisciplinary conceptual history approach, substantiated with a thorough document and legal analysis of European (...) integration and CJEU case law. I demonstrate the robustness and validity of the proposed framework by using EU asylum policy as a test case, a field where the question of solidarity is notoriously salient, which is exemplified by an area-specific expression of the concept: fair sharing of responsibility between the EU member states. Moreover, I show that the EU’s solidarity principle is necessary to maintain the bloc’s raison d’être. (shrink)