Acute-on-chronic liver failure (ACLF) is a clinical entity that is well recognized by those who care for patients with cirrhosis, however in spite of this widespread recognition, there remains little consensus with regard to definition and clinical features. While many similarities exist between ACLF and decompensated cirrhosis, there are also key differences, the implications of which are far reaching for both clinicians and patients alike. Among these differences are the possibility of a reversible component, the presence of a defined insult, prognosis, and outcomes associated with ACLF (see Fig. 1). However, for ACLF to have meaningful clinical implications, it first must be defined. If ACLF can be clearly defined and more easily recognized, then clinicians may be better able to prevent, treat, prognosticate, and counsel such patients.