Nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) are gaining increasing recognition as components of the emerging epidemic of obesity in North America and in other parts of the world. These entities are considered the hepatic manifestations of the insulin resistance syndrome and represent the spectra of fatty liver disease associated with it. All features of metabolic syndrome are associated with NAFLD/NASH, including obesity, type 2 diabetes, arterial hypertension, and hyperlipidemia in the form of elevated triglyceride levels. NAFLD/NASH can progress to liver cirrhosis and has been reported as a cause of hepatocellular carcinoma. In this review, the histopathologic features of NAFLD/NASH and differential diagnostic considerations are discussed. In addition, grading and staging schema proposed and currently in use are reviewed. Finally, other aspects for consideration by practicing pathologists, such as sampling issues, histopathologic findings after therapeutic interventions, and recurrence after liver transplantation, are addressed.