Nonalcoholic steatohepatitis (NASH) is an hepatic disorder with histologic features of alcohol-induced liver disease that occurs in individuals who do not consume significant alcohol. NASH is believed to be one of the most common explanations for abnormal liver chemistries in American adults. Risk factors for NASH include obesity, type II diabetes, hyperlipidemia, total parenteral nutrition, jejuno-ileal bypass surgery, and the use of certain drugs. However, some patients with NASH have no identifiable risk factors for the disease. Clinically, NASH is a diagnosis of exclusion that should be suspected as a cause of chronic hepatitis in patients who deny significant alcohol consumption and have negative serologic tests for congenital and other acquired causes of liver disease. The identification of fatty liver on imaging studies supports the diagnosis of NASH, which can be established definitively by liver biopsy. The latter also provides useful prognostic information since most patients with simple steatosis follow an indolent clinical course, whereas those with steatohepatitis, fibrosis, or cirrhosis are more likely to develop clinically significant complications of liver disease. Weight reduction and treatment of confounding medical conditions are the mainstays of therapy for NASH. However, there is little evidence that any of the current treatments prevent progression to more histologically advanced stages of NASH. Several experimental therapies, including treatment with bile acids, antibiotics, nutritional supplements, and antioxidants, have had anecdotal success in selected patients, but improved understanding of the pathogenesis and natural history of NASH will be required to develop generally effective therapy for the disorder.