A variety of reactions are observed after a major trauma. In the majority of cases these resolve without any long-term consequences. In a significant proportion of individuals, however, recovery may be impaired, leading to long-term pathological disturbances. The most common of these is post-traumatic stress disorder (PTSD), which is characterized by symptoms of reexperiencing the trauma, avoidance and numbing, and hyperarousal. A range of other disorders may also be seen after trauma, and there is considerable overlap between PTSD symptoms and several other psychiatric conditions. Risk factors for PTSD include severe exposure to the trauma, female sex, low socioeconomic status, and a history of psychiatric illness. Although PTSD may resolve in the majority of cases, in some cases risk factors outweigh protective factors, and symptoms may persist for many years. PTSD often coexists with other psychiatric disorders, such as depression, anxiety disorders, and substance abuse, and with physical (somatization) symptoms. There is growing evidence that PTSD does not merely represent a normal response to stress, but rather is mediated by specific neurobiological dysfunctions.