Acute stress reaction is a transient disorder of significant severity which develops in an individual without any other apparent mental disorder in response to exceptional physical and/or mental stress and which usually subsides within hours or days. Acute stress reaction was recognized by the International classification of diseases (ICD-6) in 1948. Early reactions to exceptionally stressful life events are diagnosed as acute stress reaction in ICD-102 which is kept under ‘Reactions to severe stress, and adjustment disorders’. This chapter covers the epidemiology of acute stress reaction, psychological theories, etiology, biological theories, classification, diagnosis, clinical features, stressor criterion, symptom patterns, assessment instruments, differential diagnoses, and treatment.