The euro crisis, several sovereign debt crises, the Great Recession, the refugee crisis, and Brexit have all challenged Europeans’ willingness to show solidarity with other European citizens and member states of the European Union. provides a clear theoretical framework to understand European solidarity for the first time. It offers a systematic empirical approach to determine the strength and causes of European solidarity. The authors distinguish between four domains of solidarity and test a set of theoretically derived criteria with a unique dataset to investigate European solidarity.