In leprosy, the causative bacteria, Mycobacterium leprae, will not threaten the lives of the hosts directly because they proliferate only slowly in the Schwann cells of the peripheral nerves. It is the "reactions" which give the patients irreversible morbidity through the inflammatory damages to the peripheral nerves. Physicians should be aware of the possibility of the state of the "reaction" when they examine leprosy patients. They also should be aware of the possibility of leprosy and the state of the "reaction" when they examine patients with cutaneous lesions and/or peripheral nerve disturbances, because it may be the first presenting symptom of the disease. In this review, recent advances on the issue about the reactions are discussed including pathogenesis, immunology, clinical features, pathology, treatment and prevention.