Psychiatric reactions to disasters have not received sufficient attention because it is widely accepted that human beings can endure any kind of extreme stress. A disaster is the consequence of an extraordinary event that destroys goods, kills people, produces physical or psychological harm but, above all, which overcomes the coping possibilities of the social group. Reactions to stress occur in stages, each one characterised by a specific psychological mechanisms: pre-impact, alarm, impact, recoil, post-impact and reconciliation. The reactions can be acute or chronic, among them the post-traumatic stress disorder. Symptoms include flashbacks, difficulties in remembering, avoidance of stimuli, blunting of responses, high arousal level and obsessive ruminations. Biological, psychological and social factors play a role in the pathogenesis of these disorders.