A type 1 reaction or reversal reaction is expressed clinically by inflammatory exacerbation of the skin lesions and nerve trunks, consequently leading to sensory and motor alterations. It occurs in non-polar forms of leprosy, although it can occur in a small percentage of sub-polar LL treated patients. Disabilities, deformities and morbidity, still present in leprosy, are mainly caused by these acute episodes. The recognition of reactional states is imperative for an early approach and efficient management, to avoid the emergence of disabilities that stigmatize the disease. This review aims to describe the clinical aspects, immunopathogenesis, epidemiology, histopathological features and therapeutics of type 1 reactions.