The clinical presentation of PTSD varies. It is not entirely clear what is seen as a traumatic event and what is not. A life-threatening illness or medical condition is not seen as trauma but medical incidents can qualify as traumatic events (e.g. waking up during surgery), same as a medical catastrophe in one’s child. Intrusive recollection is not the same as depressive rumination. Intrusive recollection applies to involuntary and intrusive distressing memories. It can be short (e.g. flashback) but can lead to prolonged stress and heightened arousal. In PTSD, there often is a heightened sensitivity to threats. Developmental regression (e.g. loss of language) may occur in children. PTSD can lead to difficulties in regulating emotions or maintaining stable interpersonal relationships.The lifetime prevalence of PTSD is 8.7% in the United States and the twelve-month prevalence is 3.5%. These estimates are lower in many other countries (e.g. European countries). Different groups have different levels of exposure to traumatic events. The conditional probability of developing PTSD following a similar level of exposure may differ between groups. Cultural syndromes (e.g. ataques de nervosia) may influence the expression of PTSD. The risk of onset of PTSD and severity may differ across cultural groups as a result of:Variation in the type of traumatic exposure (e.g. genocide).The meaning attributed to the traumatic event.The ongoing sociocultural context.Other cultural factors.PTSD appears to be more severe if the traumatic event is interpersonal and intentional (e.g. torture). The highest PTSD rates are found among rape survivors (1), military combat and captivity survivors (2) and ethnically and politically-motivated internment and genocide survivors (3). Young children and older adults are less likely to show full-threshold PTSD.The symptoms and relative predominance of symptoms may vary over time. Symptom recurrence and intensification...