Insulin resistance (IR) and metabolic syndrome are associated with the pathogenesis and progression of non-alcoholic fatty liver disease (NAFLD). It encompasses a spectrum of hepatic pathology, ranging from simple steatosis in its most benign form (fatty liver), going through nonalcoholic steatohepatitis (NASH) in its intermediate form, until cirrhosis and hepatocellular carcinoma, its most advanced forms. This book presents current research on the background of fatty acid oxidation; hepatic steatosis and its progression to NASH and cirrhosis; fibrosis and myofibroblastic cells differentiation; methods of diagnosis; clinical features and laboratory tests and, finally, rodent NAFLD and NASH models.