Background: Liver resection represents the first choice of treatment for primary and secondary liver malignancies, offering the patient the best chance of long-term survival. The extensive use of major hepatectomy increases the risk of post-hepatectomy liver failure (PHLF), which is associated with a high frequency of postoperative complications, mortality and increased length of hospital stay. Aims: The aim of this review is to investigate the different risk factors related to the occurrence of PHLF and to identify the limits for a safe liver resection in patients with normal liver and injured liver (cirrhosis, cholestasis, steatosis and post-chemotherapy liver injury). Methods: A literature search was undertaken in PubMed and related search engines, looking for articles relating to hepatic failure following hepatectomy in normal liver or injured liver. Results: In spite of improvements in surgical and postoperative management, the parameters determining how much liver can be resected are still largely undefined. A number of preoperative, intraoperative and postoperative factors all contribute to the likelihood of liver failure after surgery. The safe limits for liver resection can be estimated from the data of the literature for patients with normal liver and for those with different types of liver injury. Conclusions: Preoperative assessment that includes evaluation of liver volume and function of the remnant liver is a mandatory prerequisite before major hepatectomy. The critical residual liver volume for patients able to predict PHLF is mainly related to the presence of pre-existing liver disease and liver function. Among patients with normal liver, the limit for safe resection ranges from 20 to 30% future remnant liver of total liver volume. In patients with injured liver (cirrhosis, cholestasis or steatosis), preoperative assessment of the risk of PHLF should include future remnant liver volumetry and accurate liver function evaluation, including different dynamic liver function tests.