Psychotropic medication use is exclusionary at study entry; Veterans who have been previously treated with a stable regimen may not be enrolled until a minimum of five half-lives have elapsed since the Veteran last took any psychotropic medications. (Sleep aids such as trazodone (up to 50 mg/day) or zolpidem (up to 10 mg/day) may be offered to Veterans with insomnia on an as-needed basis at the onset of the study.) Major depression and other anxiety disorders are not exclusionary since they frequently co-occur with PTSD and it remains unclear whether their presence represents true comorbidity, symptom overlap, or severe illness. Since mild head trauma is common in the military population in general, excluding such participants would diminish the generalizability of the sample; participants with severe traumatic brain injury (TBI), defined as an extended period of unconsciousness or amnesia following injury, will be excluded. The Ohio State University (OSU) TBI Identification Method will be used to assess lifetime history of TBI. This method first establishes all significant injuries in one's life and then determines if a TBI may have occurred based on whether the participant experienced a loss of consciousness (LOC) and, if so, for how long. A person is said to have a mild TBI if LOC does not exceed 30 minutes for any injury, a moderate TBI if LOC is between 30 minutes and 24 hours, and a severe TBI if LOC exceeds 24 hours. The OSU TBI assessment will be administered by a trained rater during the psychiatric evaluation conducted at screening.