Posttraumatic stress disorder (PTSD) is a highly prevalent disorder in both clinical and community populations. This article reviews current knowledge about PTSD in order to assist clinicians in the diagnosis and treatment of reactions to traumatic life events. First, research findings are presented, followed by guidelines for the assessment of trauma and PTSD. Topics discussed include epidemiology, course, and comorbidity, as well as information processing and psychobiology. The review is limited to information about PTSD in adults, although some of the material may generalize to child and adolescent populations. © 2002 Wiley Periodicals, Inc. J Clin Psychol/In Session 58: 877–889, 2002.