Background: Acute liver failure (ALF) is characterized by a sudden loss of hepatic function due to hepatocyte cell death and dysfunction in previously healthy individuals. The clinical presentation of ALF is associated with coagulopathy (international normalized ratio ≥1.5) and hepatic encephalopathy, although the latter may be less pronounced. Without appropriate and timely intensive care or liver transplantation (LTx), ALF will result in multi-organ failure and death. Various causes may induce ALF, with acetaminophen (APAP) intoxication and acute hepatitis B infection as most common causes in industrialized countries. While conventional terminology discerns acute, acute-on-chronic and acute-on cirrhosis liver failure, some chronic liver diseases (i.e. autoimmune hepatitis (AIH), Wilson's disease) can remain undiagnosed until an initial presentation as ALF. Key Messages: Upon definite diagnosis of ALF, the underlying cause must be identified, since etiology affects prognosis and clinical management. Individual prognosis should be evaluated with one of various available scoring systems. Most widely used are Model for End-Stage Liver Disease, the King's College Criteria and the Clichy criteria. Other markers, that is, cell death markers, lactate or thyroid status, may improve diagnostic accuracy of classic scores, though routine use of these is not yet established. Etiology-specific treatment under intensive care should be performed, if possible (APAP and amanita intoxication, acute viral hepatitides and AIH). LTx is the only curative option for other causes, unknown reasons of ALF or when etiology-specific therapy fails. In ambiguous cases, that is, suspected drug induced ALF or AIH, co-infection with hepatitis E virus should be tested, as this might be more common than it is currently supposed to be. Conclusions: Despite major improvements in clinical management of ALF patients, a significant proportion of ALF cases remains without clear identification of the underlying cause or unrecognized multiple causes. In depth analyzes of ambiguous ALF cases is warranted to further improve clinical management.