Until recently, our understanding of posttraumatic stress disorder (PTSD) relied almost entirely on studies of war veterans and disaster victims. A handful of epidemiologic studies have now been conducted that investigate the natural course of PTSD as it occurs in the general population. Estimates of PTSD prevalence have tended to vary according to the diagnostic criteria used to define the disorder, assessment procedures, sample characteristics, and the definition of qualifying traumatic events. This article reviews key findings from these studies to provide insight into the burden of PTSD in the general population. Possible reasons for the observed difference in lifetime prevalence of PTSD between the sexes (a female-to-male lifetime prevalence ratio of 2:1 is typically reported) and factors thought to be associated with an increased risk for the disorder after exposure to trauma are reviewed.