The exposure to a traumatic event may result in posttraumatic stress disorder (PTSD), which is characterized by a complex symptomatology, clustered into three groups of symptoms, i.e. intrusive memories,avoidance behavior, and hyperarousal. Since PTSD is a stress reaction, alterations of stress-responses of neurobiological systems have been examined in patients suffering from PTSD. The investigation of biological parameters refers to the hypothalamic-pituitary-adrenal axis (HPA), studies of the noradrenergic and the endogenous opiate system as well as psychophysiological and neuroi-maging studies. Besides others, the observed biological dysregulations refer to hypocortisolism with an enhanced negative feedback of the HPA axis, enhanced noradrenergic activity, and neuroanatomical changes. To elucidate the specifity of this findings for PTSD,the dysregulations will be discussed with reference to findings in major depression and somatoform disorders.