For a PTSD diagnosis, a person must have 1 or more of the following Criterion B symptoms:

• Recurrent, involuntary, and intrusive distressing memories of the trauma(s).
• Recurrent distressing dreams in which the content and/or affect of the dream are related to the trauma(s).
• Dissociative reactions (e.g., flashbacks) in which the individual feels or acts as if the trauma(s) is recurring.
• Intense or prolonged psychological distress at exposure to internal or external cues that symbolize or resemble an aspect of the trauma(s).
• Marked physiological reactions to internal or external cues that symbolize or resemble an aspect of the trauma(s).

The Criterion B or intrusion symptoms are perhaps the most distinctive and readily identifiable symptoms of PTSD. The memory of the trauma can intrude into the daily life of a PTSD sufferer in the form of daytime images of the event, traumatic nightmares, and/or vivid reenactments known as flashbacks (a type of dissociative reaction). Emotional and physiological reactions may also be triggered by explicit trauma reminders. The intrusion symptoms of PTSD are distinguished from "normal" remembering of past events by the fact that memories of the trauma(s) are unwanted, occur involuntarily, elicit distressing emotions, and disrupt functioning and quality of life.