The chapter of Markus Kotzur examines solidarity as a legal concept. Whereas the Treaty of Lisbon contains manifold references to the notion of solidarity, Kotzur argues that a precise meaning of or conceptual framework for this notion is anything but clear. The question of which overall expectations, moral obligations, political assumptions and normative forces can be attributed to “solidarity” needs, therefore, to be addressed. Against this backdrop, the chapter aims, firstly, to introduce conceptual approaches to solidarity on the national, regional and international planes; secondly, to identify text-based legal substantiations of solidarity within the EU and beyond; and, finally, to frame solidarity as a constitutional paradigm. In doing so, the paper follows a leitmotif once expressed by Jacques Delors in the saying “solidarity mechanisms are not based on pure generosity, but on enlightened self-interest”.