Abstract
A coherent intellectual structure for social movement studies has recently been emerging over a range of theoretical and empirical studies. This structure counterposes ‘within social movements’ a diverse range of collective actions against the unity imposed by a collective identity. However, theorisations of this collective identity have so far failed to address the contradiction between structure and agency. A definition of collective identity for social movements that is not caught in the structure/agency divide is proposed by defining the appropriate level of abstraction for such a definition, defining why movements are unified and then how.