Experiences in crowds and protests can lead to psychological changes which are sustained by group processes of support and communication. Riots can spread across locations via shared identity and influence among participants or via empowerment in relation to perceived police weakness. Social support among survivors is common in mass emergencies and can be facilitated by informed action by professional groups. Pedestrian behaviour varies based on whether they are part of a physical crowd, in which people are simply together in the same space, or a psychological crowd, in which people share a social identity. Mass gatherings can be the basis of improvements in health and changes in attitudes, but they can also be a source of risk through the same social identity processes.