This chapter distinguishes solidarity as a legal concept (LS) from solidarity as a social practice (SP). It matters for our understanding of the law to reflect on how, when and why law is able to interact with solidaristic practices. Section 1.1 explores the distinction. Section 1.2 stresses the ubiquity of solidarity in the law, from the traditional private law understanding of obligatio in solidum, to solidarity as a cohesive social force, to solidarity as a source of state duties. Section 1.3 shows that, despite its omnipresence, solidarity is an underinvestigated legal concept. Section 1.4 offers a typology of interactions between SP and the law, to show the many ways in which legal scholars may relate to SP. I list several types of interaction, and object to one. I argue that law cannot command us to act solidaristically since solidarity presupposes an intimate form of identification with others. But law may disrupt solidarities, sometimes in morally justified ways; it may compensate for the failing solidarity, recognizing and integrating it; and it may foster solidarity by its status-generative function, albeit merely in an indirect and not often controllable way.