Most people experience at least one potentially traumatic event (PTE) during their life. Many will develop only transient distress and not a psychological illness. Even the most inherently horrific event does not invariably lead to the development of a psychological disorder while an individual with sufficient vulnerability may develop post-traumatic stress disorder (PTSD) after what appears be an event of low magnitude. The diagnosis of PTSD differs fro most psychiatric disorders as it includes an aetiological factor, the traumatic event, as a core criterion. The DSM 5 core symptoms of PTSD are grouped into four key symptom clusters: re-experiencing, avoidance, negative cognitions and mood, and arousal. Symptoms must be present for at least one month and cause functional impairment. PTSD patients can avoid engaging in treatment and assertive follow-up may be necessary.