Posttraumatic stress disorder (PTSD) is a fairly common psychiatric disorder that is associated with a lifetime prevalence of approximately 9% in the United States. In light of recent war and terrorist activity worldwide, it is likely that increased numbers of individuals will be exposed to severe or life-threatening trauma, and the incidence of PTSD may be even higher than previously indicated in epidemiologic studies. PTSD may develop after exposure to a traumatic event in which the individual experienced, witnessed, or was confronted by either actual or threatened loss of life or serious injury. Patients with PTSD often reexperience intrusive recollections of the event in ways that are highly distressing and may be described as reliving the memory. Not surprisingly, symptoms of avoidance are noted because individuals with PTSD often wish to escape recollections (thoughts, feelings, conversations, places) related to the trauma. Patients also experience symptoms of hyperarousal associated with difficulty concentrating or exaggerated startle response. Notably, individuals who develop PTSD represent only a subset of those exposed to trauma. It is of interest why certain individuals are at risk for development of PTSD after traumatic exposure, whereas others appear to be more resilient to the effects of trauma. Studies suggest that previous exposure to trauma and intensity of the response to acute trauma may affect the development of PTSD. In addition, however, neuroendocrine changes, such as lower cortisol levels, also may influence formation and processing of traumatic memories and may be associated with the underlying pathology of PTSD.