PTSD has been recognized for centuries, but its diagnosis is still being refined. PTSD currently refers to a person who has reacted with "intense fear, helplessness, or horror" to a major (or minor) trauma by developing (1) intrusive, re-experiencing symptoms; (2) avoidance responses to evidence of the trauma and generalized psychological numbing and isolation; and (3) widespread physiologic arousal. It can last for 1 to 3 months (acute) or more than 3 months (chronic) or develop for the first time at least 6 months after the trauma (delayed onset). Many of the long-lived symptoms of PTSD seem to have a biologic basis, even though the condition has environmental roots: It is one of the rare conditions in psychiatry for which one can create an animal model. Most patients improve, even if the trauma was severe, but some do not. With serious trauma, the lifetime prevalence rate for PTSD from most causes tends to be about 30% of those exposed, whereas the current prevalence after several years is usually below 10%. The symptoms wax and wane over months and years but can return in full force with retraumatization. A few patients, however, often with appropriate therapy, can turn PTSD into a growth experience. PTSD often grades into, or is comorbid with, disorders such as generalized anxiety disorder, panic disorder, major depression, chronic dysthymia, alcoholism, and somatoform disorders. Its development is determined both by the nature and power of the stressor as well as the intrinsic vulnerability of the patient and the treatment received by the patient after (usually soon after) the traumatic event. Although much is known about PTSD, even more remains unclear, and we can expect further refinements in our understanding of this disorder in the years to come.