This review provides an overview and new integration of recent research that has formed the basis of a social identity explanation of supportive collective behaviour among survivors in emergencies and disasters. I describe a model in which a sense of common fate in the emergency or disaster is the source of an emergent shared social identity among survivors, which in turn provides the motivation to give social support to others affected. In addition, by drawing on the concept of relational transformation in psychological crowds, I show how an emergent shared social identity can engender a range of further behavioural and cognitive consequences that contribute to collective self-organisation in emergencies, including increases in expected support, coordination of behaviour, and collective efficacy. It will be argued that the model can been applied to explaining how potentially dangerous crowd events avoid disaster: shared social identity operates as the basis of spontaneous self-organisation in these cases, as in many emergencies and disasters.