The most widely accepted definition of ALF includes evidence of coagulation abnormality, usually an INR ≥ 1.5, and any degree of mental alteration (encephalopathy) in a patient without preexisting cirrhosis and with an illness of < 26 weeks duration. Terms used signifying length of illness such as hyperacute, acute and subacute are not particularly helpful since they do not have prognostic significance distinct from the cause of the illness. The outcomes of patients with ALF vary greatly according to etiology, and the etiology of ALF varies markedly by geographical region. In Korea, about 90% of ALF are associated with etiologies that usually result in poor outcomes, including HBV infection and herbal remedies. The main causes of death in patients with ALF are increased intracranial pressure, systemic infection, and multi-organ failure. Recent advances in the intensive care of patients with ALF have contributed to a marked improvement in their overall survival. The necessity of emergency adult to adult living-donor liver transplantation (LDLT) is more emphasized in this country not only because of the limited supply of organs from deceased donors but also because of the common etiologies associated with extremely poor transplant-free survival rate.