Liver steatosis or nonalcoholic fatty liver disease (NAFLD) is a progressive disorder characterized by the fat accumulation in the parenchyma of the liver. The progressive form of NAFLD called nonalcoholic steatohepatitis (NASH) is the advanced condition that shows different grades of fibrosis, which could progress to cirrhosis, hepatocellular carcinoma and liver-related morbidity and mortality. Bariatric surgery gains attention as a potential treatment for liver steatosis because different studies revealed an improvement in 90% of cases of fatty liver after surgery. The improvement occurred within the first year and persisted up to five years. Pathogenic mechanisms that are involved in the evolution steatosis are intimately connected with insulin resistance, type 2 diabetes (T2D) and the metabolic syndrome. Bariatric surgery is a powerful treatment to ameliorate all these conditions. The aim of this book is to update the actual evidence about liver steatosis, non-alcoholic fatty liver disease and non-alcoholic steatohepatitis in morbidly obese patients, the diagnostic methods and the evolution after the different techniques of bariatric surgery.