This prompted the introduction of post-traumatic stress disorder (PTSD) as a diagnostic category in DSM-III. It was thus recognized that traumatic events such as combat, rape, man-made or natural disasters give rise to a characteristic pattern of psychological symptoms. The diagnostic criteria specified the experience of a traumatic event as a necessary condition for the diagnosis. ICD-10 emphasized the causal role of traumatic stressors in producing psychological dysfunction even more clearly, in that a specific group of disorders, ‘reaction to severe stress, and adjustment disorders\', was created. This chapter covers the epidemiology of PTSD, PTSD with psychiatric comorbidity, etiology of PTSD, hypothesis for PTSD, clinical features of PTSD, classification of PTSD, diagnosis of PTSD, differential diagnosis of PTSD, factors affecting recovery from trauma, and trauma-focused psychological treatment.