This article reviews concepts that help synthesize the data on posttraumatic stress disorder (PTSD), a very complex condition in terms of its etiology, psychobiology, epidemiology, comorbidity, and treatment. At least four neurobiologic systems are involved in PTSD: the catecholamine, the hypothalamic-pituitary-adrenocortical, the thyroid, and the endogenous opioid systems. Six other systems are probably or possibly implicated as well. The avoidance and hyperarousal of PTSD distort the patient's appraisal of the world. The symptoms of PTSD can be understood through models of learning and memory, which form the basis of behavioral treatments. The concepts of tonic and phasic alteration and of allostasis versus homeostasis also shed light on PTSD. In addition to PTSD, there may be other identifiable posttraumatic syndromes that might be diagnosed separately, such as "complex" PTSD. Cross-cultural issues may also affect clinical phenomenology and thereby confuse the diagnosis. Comorbid disorders may actually be clues to subtypes of PTSD. The fact that victims of PTSD are also more vulnerable to medical illnesses makes a closer relationship with primary care providers and other specialists mandatory. New approaches to prevention, treatment of chronic PTSD, psychotherapy, pharmacotherapy, and research hold promise of an improved prognosis for patients with PTSD.