Posttraumatic stress disorder (PTSD) can be characterized as a failure of recovery caused, in part, by a failure of fear extinction after trauma. By studying the process of extinction, we can be informed regarding the etiology and maintenance of PTSD. The normal response to trauma in humans includes a set of predictable reactions including reexperiencing, avoidance, and hyperarousal that typically extinguish in the days and weeks after the trauma. In the majority of people exposed to trauma, these responses extinguish over time. However, in a substantial minority, extinction fails and these persisting responses become the symptoms of PTSD. Therefore, one of our fundamental hypotheses is that PTSD is a disorder caused in part by the failure of extinction of predictable posttraumatic physiological and psychological reactions. The most empirically validated treatments for PTSD involve exposure of the patient to trauma-related cues in the absence of danger that then lead to the extinction of these reexperiencing, avoidance, and arousal symptoms. There is also mounting evidence that individuals with PTSD are more resistant to extinction. Regarding early interventions with traumatized individuals, there is mounting evidence that some early one-time interventions actually may impede extinction, whereas interventions delivered in more than one session, at least several weeks after the trauma, to individuals continuing to experience above average reactions, generally are effective in preventing the development of PTSD. Thus, there appears to be an interaction between timing of the intervention, number of intervention sessions, and either arousal level and/or risk status in determining whether the intervention will be helpful, harmful, or neutral.