Since trauma arises from an inescapable stressful event that overwhelms people's coping mechanisms, it is uncertain to what degree the results of laboratory studies of ordinary events are relevant to the understanding of traumatic memories. This paper reviews the literature on differences between recollections of stressful and of traumatic events. It then reviews the evidence implicating dissociation as the central pathogenic mechanism that gives rise to posttraumatic stress disorder (PTSD). A systematic exploratory study of 46 subjects with PTSD indicated that traumatic memories were retrieved, at least initially, in the form of dissociated mental imprints of sensory and affective elements of the traumatic experience: as visual, olfactory, affective, auditory, and kinesthetic experiences. Over time, subjects reported the gradual emergence of a personal narrative that can be properly referred to as "explicit memory." The implications of these findings for understanding the nature of traumatic memories are discussed.