This review suggests an explanation for the finding that trauma is a necessary but insufficient condition for the development of posttraumatic stress disorder. Predispositions affecting vulnerability to stress can help to account for discrepancies between traumatic exposure and pathological outcome. Such predispositions may be rooted in personality traits that shape the cognitive processing of stressful events, but they are also influenced by both prior life experiences and social supports. All of these observations are consistent with a biopsychosocial model of posttraumatic stress disorder.