This article discusses the biomedical and the social constructionist models applied to response to trauma, presents the prevalence and the etiology of post-traumatic stress disorder (PTSD), and describes its biological and psychological correlates in children and adults. It concludes that future research might benefit from investigating factors that may protect people who have been exposed to an event likely to be traumatic from presenting with PTSD symptoms, and factors that may affect the longitudinal course of PTSD and treatment effectiveness.