Nonalcoholic fatty liver disease (NAFLD) is a diagnostic consideration among patients with asymptomatic elevated aminotransaminases, patients with radiologic findings of hepatic fatty infiltration, or occasionally in the patient with "cryptogenic" cirrhosis. The diagnosis of NAFLD requires evidence of fatty infiltration of the liver in the absence of excessive alcohol ingestion. Clinical evaluation should examine for metabolic risk factors (central obesity, glucose intolerance, hypertension, hypertriglyceridemia, and low HDL cholesterol), which are suggestive but not specific for the diagnosis of NAFLD. Secondary causes of NAFLD, such as medications and intestinal bypass surgery, should be excluded as management of these conditions may differ. Confirmation of hepatic steatosis can usually be done by imaging studies, although occasionally liver biopsy is required. Among suspected NAFLD patients with chronically elevated aminotransaminases, clinical evaluation and serological testing should be performed to exclude other causes of chronic liver disease. Liver biopsy is required to stage fibrosis and distinguish between nonalcoholic steatohepatitis and steatosis. This is valuable for providing prognosis, excluding other liver disease, monitoring response to therapy or evaluating disease progression over time. Clinical features, particularly diabetes, obesity, and older age, can aid in stratifying patients at risk for advanced fibrosis but are not sufficiently accurate to replace liver biopsy.