Patients with cirrhosis who are hospitalized for an acute decompensation (AD) and also have organ failure(s) are at high risk of short-term death. These patients have a syndrome called Acute-on-Chronic Liver Failure (ACLF). ACLF is now considered as a new syndrome that it is distinct from "mere" AD not only because of the presence of organ failure(s) and high short-term mortality but also because of younger age, higher prevalence of alcoholic etiology of cirrhosis, higher prevalence of some precipitants (such as bacterial infections, active alcoholism), and more intense systemic inflammatory response. ACLF is a new syndrome also because severe sepsis or severe alcoholic hepatitis do not account for 100% of the observed cases; in fact, almost 50% of the cases are of "unknown" origin. In other words, severe sepsis, severe alcoholic hepatitis and ACLF of "unknown origin" are subcategories of the syndrome.