Recently, acute-on-chronic liver failure has been recognized as a specific and unique clinical form of liver failure (usually related to acute insult) in patients with previously known or unknown compensated chronic liver disease. Its main feature is the reversibility, and high short-mortality due to multiorgan failure (MOF) in the absence of liver support system devices and/or liver transplantation. This article aims to introduce the definition and better understanding of this newly developed and unique profile of liver failure.