The liver is the first organ to receive nutrients that enter the body via the intestines after a meal, and this organ plays a pivotal role in energy metabolism. Non-alcoholic fatty liver disease (NAFLD) is among the most common causes of chronic liver disease in the world and is now considered to be a component of metabolic syndrome. A wide spectrum of histological changes has been observed in NAFLD, ranging from simple steatosis, which is generally non-progressive, to nonalcoholic steatohepatitis (NASH), liver cirrhosis, liver failure, and sometimes even hepatocellular carcinoma. This book reviews recent evidence concerning insulin resistance and NAFLD/NASH as a step towards a comprehensive understanding of the pathophysiology and treatment of NAFLD/NASH.