Psychophysiological research in trauma-exposed populations has provided objective data supporting the validity of the post-traumatic stress disorder (PTSD) diagnostic concept. Consistent with a conditioning model, PTSD patients show specific increased peripheral physiological responding to audio-visually and imaginally presented stimuli symbolizing or resembling the etiologic traumatic event. PTSD patients respond to startling stimuli with larger autonomic and electromyographic responses, especially under threat conditions. Electroencephalographic event-related potential (ERP) response abnormalities in PTSD include reduced P2 amplitude at high stimulus intensities, impaired P1 habituation, and attenuated P3 amplitude to target auditory stimuli. However, larger P3 and N1 amplitude responses and shorter P3 and N1 latencies have been reported in PTSD subjects in response to trauma-related stimuli. These ERP findings suggest sensory, cognitive, and affective processing abnormalities in PTSD. Polysomnographic sleep studies have revealed increased awakenings, reduced sleep time, and increased motor activity, or in some cases, paradoxical deepening of sleep. There is also evidence for increased phasic eye movement activity during rapid eye movement (REM) sleep and disrupted REM continuity in PTSD. Psychophysiological studies are offering valuable insights into the pathophysiology of this important neuropsychiatric condition.