 Assessment: PTSD subjects will be rated for current psychopathology with the Clinician Administered PTSD Scale and the Mississippi Scale for Combat-Related PTSD. In addition, PTSD subjects and combat controls will be rated for severity of combat exposure with the Combat Exposure Scale. In all subjects, childhood trauma will be assessed with the Early Trauma Inventory. Current depression and anxiety symptoms will be assessed using the Hamilton Depression and Hamilton Anxiety scales, respectively. Attention and distractibility will be assessed by means of the Continuous Performance Test, and motor function will be assessed with the Finger Tapping Test. Cognitive function will be assessed with the Benton Visual Retention Test, the California Verbal Learning Test, Wechsler Abbreviated Scale of Intelligence, Wechsler Memory Scale, Selective Reminding Test, and the Trails Test. Results of these tests will be used as covariates in the analysis of conditioning data in order to control for potential group differences in attention or memory. Total time of assessment should be about four to five hours. 