PTSD can be a chronic, devastating disorder for which treatment is only partially effective. For some, this disorder progressively worsens over time and appears to affect nearly every aspect of life, including work, interpersonal relationships, physical health, and view of self. Although generally understood as a psychological disorder, PTSD also may be viewed from a biologic perspective. There is now accumulating evidence to suggest that severe psychological trauma can cause alterations in the organism's neurobiologic response to stress even years after the original insult. Long-standing alterations in the biologic response to stress may contribute to a number of complaints commonly expressed by patients with PTSD. For example, increased sensitivity and sensitization of the noradrenergic system may leave the individual in a hyperaroused, vigilant, sleep-deprived, and, at times, explosive state that worsens over time. Being irritable and on edge makes it difficult to interact with family members, friends, coworkers, and employers. To quiet these symptoms of hyperarousal, PTSD patients often withdraw and use substances, particularly central nervous system depressants, that suppress peripheral and central catecholamine function. Alterations in other neurobiologic systems may further contribute to multiple symptoms, such as intrusive memories, dissociation phenomena, and even numbing. Characterization of the biologic underpinnings of PTSD relies to a large degree on available neurobiologic technology. Much of what has been discussed in this article has grown out of advances in physiologic, hormone, and receptor assay methodology. With further advances in neurobiologic technology, in areas such as brain imaging, it soon will be possible to better delineate acute and long-term stress-induced changes in central and peripheral nervous system functioning. Undoubtedly a far richer, more complex understanding of neurobiologic responses and alterations will emerge in the near future. It is believed that an improved neurobiologic understanding will facilitate the development of more specific, effective treatments for individuals who have been severely traumatized.