The nature of acute and chronic stress is explored, including analysis of three different ways of categorizing the duration of a stressful episode: the duration of the physical stressor, the duration of threat perception or demand, and the persistence of response. Of particular interest here are situations characterized by brief stressor exposure but long-term threat perception and/ or stress response. Traumatic events are ordinarily very brief but frequently give rise to chronic threat and stubborn response patterns. In many of these cases, distress clearly outlives the event and the “normal” postevent recovery period. Possible mechanisms for such situations are discussed, as are implications for the study of traumatic stress.