This chapter examines the concept of solidarity, especially its existence outside the law, as a mechanism of cohesion. Three features are characteristic of this mechanism. First, solidarity mediates between the community and the individual. Second, as a result of solidarity, unity is created. Third, solidarity carries with it positive obligations, requiring individuals to act in support of, and in conformity with the group. Apart from these three general characteristics, solidarity is a multifaceted concept, with differing implications depending on the context in which it features. To understand these implications the chapter distinguishes between three kinds of solidarity: ‘social solidarity’, ‘welfare solidarity’ and ‘oppositional solidarity’. After a short discussion of each, the chapter pays special attention to social solidarity. On the basis of Aristotle’s notion of ‘friendship’, Rousseau’s ‘social contract’, Durkheim’s ‘mechanical’ and ‘organic’ solidarity and Parsons’s understanding of solidarity as a normative obligation, it analyses the concept’s roots and evolution over time.