Acute-on-chronic liver failure (ACLF) is a clinical syndrome characterized by acute decompensation of liver cirrhosis, organ failure and high short-term mortality (20-80% at one month). The main precipitants are infections and excessive alcohol intake, and the mechanistic features include a high level of systemic inflammation, macrophage activation and liver injury. The severity of ACLF is graded according to the number and extent of organ failures. Prognostic scores help predict mortality and support decisions on intensive treatment or futility.