This chapter explores the diversity of collectives, making the link with the usual notions of the social sciences (classes, organisations, etc.). The generic notion of collective corresponds to what is common to classic social science notions such as organization, social class or group. The relations between persons and collectives take a variety of forms, from simple membership to a situation in which the person is seen as a collective (a monad), to the more ambiguous case where he or she has links with certain members of the collective. A relation between two collectives is a formalized reciprocal commitment. Explicit collectives are structured by specific coordination resources that form the basis of three types of internal order, present in variable distributions: polarization in relation to an exterior; internal deliberation; and equilibrium between participants.