Posttraumatic stress disorder (PTSD) is a well-defined clinical syndrome that develops in individuals who have witnessed or been exposed to an event that involves a direct threat to life or physical and/or psychological integrity.

While numerous studies indicate that PTSD will develop in 15% to 25% of trauma victims, time-limited responses develop in a larger portion of victims during the first 48–72 hours (acute stress reaction) and to a lesser extent over the first 4 weeks (acute stress disorder). Many of those who suffer from acute posttraumatic symptoms eventually recover and overcome the consequences of the traumatic event. However, ∼30% to 50% of those with acute stress disorder will eventually develop PTSD. It is obvious that some individuals are vulnerable to the adverse effects of trauma while others have neurobiological and psychological resources that make them resilient to the long-term impact of traumatic exposure. The identification of risk factors and early predictors for PTSD is thus of major importance for identifying those at risk and for initiating therapeutic interventions aimed to reduce long-term morbidity and suffering.