In response to growing national concern about the number of veterans who might be at risk for posttraumatic stress disorder (PTSD) as a result of their military service, the Department of Veterans Affairs (VA) asked the Institute of Medicine (IOM) to conduct a study on the diagnosis and assessment of, and treatment and compensation for PTSD. An existing IOM committee, the Committee on Gulf War and Health: Physiologic, Psychologic and Psychosocial Effects of Deployment-Related Stress, was asked to conduct the diagnosis, assessment, and treatment aspects of the study because its expertise was well-suited to the task. The committee was specifically tasked to review the scientific and medical literature related to the diagnosis and assessment of PTSD, and to review PTSD treatments (including psychotherapy and pharmacotherapy) and their efficacy. In addition, the committee was given a series of specific questions from VA regarding diagnosis, assessment, treatment, and compensation. Posttraumatic Stress Disorder is a brief elaboration of the committee’s responses to VA’s questions, not a detailed discussion of the procedures and tools that might be used in the diagnosis and assessment of PTSD. The committee decided to approach its task by separating diagnosis and assessment from treatment and preparing two reports. This first report focuses on diagnosis and assessment of PTSD. Given VA’s request for the report to be completed within 6 months, the committee elected to rely primarily on reviews and other well-documented sources. A second report of this committee will focus on treatment for PTSD; it will be issued in December 2006. A separate committee, the Committee on Veterans’ Compensation for Post Traumatic Stress Disorder, has been established to conduct the compensation study; its report is expected to be issued in December 2006.