POST-TRAUMATIC STRESS DISORDER (PTSD) is a common and often chronic and disabling anxiety disorder that can develop after exposure to highly stressful events characterized by actual or threatened harm to the self or others. This is the first of two articles summarizing the nature and treatment of PTSD and the associated condition of acute stress disorder (ASD). The present article presents the diagnostic criteria for PTSD and ASD, summarizes the epidemiology of exposure to trauma and resulting PTSD/ASD, discusses implications of these data for assessment and treatment, and provides a summary of several useful assessment instruments. A companion paper to be published in a future issue of Psychiatry 2005 will provide a summary of empirically supported treatments, both psychological and pharmacological, for PTSD and ASD.