Acute-on-chronic liver failure (ACLF) is a newly defined syndrome in patients with liver cirrhosis characterized by acute hepatic decompensation (jaundice, ascites, hepatic encephalopathy, bacterial infection and gastrointestinal bleeding), single or multiple organ failure and a high mortality (>15% within 28 days). The affected organ systems include not only the liver but also the circulation, lungs, kidneys, brain and/or coagulation. Pathophysiologically decisive is an uncontrolled inflammation that is induced by specific triggers and on the basis of previously (possibly not diagnosed) compensated as well as already decompensated liver cirrhosis leads to a severe systemic clinical syndrome, ACLF. The course during the first 72 h is decisive for the prognosis. In addition to treatment of the respective organ or system failure, the underlying triggers should be quickly identified and if necessary specifically treated. Often, however, these cannot (no longer) be determined with any certainty, in particular recent alcohol consumption as well as bacterial and viral infections play an important role. A specific treatment for the ACLF is (currently) not established. Some experimental approaches are currently being tested, including administration of granulocyte colony-stimulating factor (GCSF). Additionally, suitable patients should be presented to a liver transplantation center in a timely manner.