Obesity-related liver disease, particularly nonalcoholic fatty liver disease and its more severe form nonalcoholic steatohepatitis, is being increasingly recognized as a common liver disease in developed countries and represents hepatic manifestation of the metabolic syndrome. Nonalcoholic steatohepatitis results in progressive fibrosis, cirrhosis, and end-stage liver disease, with its increased incidence of hepatocellular carcinoma. Liver biopsy remains the gold standard in detection, evaluation, staging, and grading of nonalcoholic steatohepatitis. Liver biopsy also provides an important tool to detect concomitant liver disease that may accelerate the progression of steatohepatitis in these patients. Surgical or drug-induced weight loss has been documented to reduce the amount of inflammation, to induce regression of fibrosis, and, in some cases, to cause a reversal of cirrhosis in obese patients with steatohepatitis. Pretreatment or intraoperative liver biopsy should be performed to assess the presence of steatohepatitis, and stage and grade of steatohepatitis. This article will discuss obesity-related liver diseases, focusing on pathologic features in liver biopsies from obese patients, obesity-related hepatocellular carcinoma, and the effect of weight loss treatment on histopathology.