N 'onalcoholic fatty liver disease (NAFLD) includes a spectrum of hepatic pathology that resembles alcohol-induced liver disease but develops in individuals who are not heavy drinkers. NAFLD is likely to be the most common cause of chronic liver disease in many countries and may also potentiate liver damage induced by other agents, such as alcohol, industrial toxins, and hepatotrophic viruses. The lack of specific and sensitive noninvasive tests for NAFLD limit reliable detection of the disease. Often, NAFLD is diagnosed presumptively when imaging studies suggest hepatic steatosis or when liver enzyme elevations are noted in overweight or obese individuals with no other identifiable reason for liver disease. NAFLD is strongly associated with insulin resistance and dyslipidemia; however, whether it is a cause or a consequence of these other conditions in not clear. The natural history of NAFLD is also uncertain. Nevertheless, discrepancies between the apparently high prevalence of NAFLD in the population and the generally low prevalence of clinically significant liver disease, as well as the relatively low representation of NAFLD patients among liver transplant populations, have generated considerable skepticism about the clinical importance of NAFLD. The latter has reduced enthusiasm for aggressive diagnosis, treatment, and follow-up of patients with NAFLD. This review of the clinical literature and selected basic information about NAFLD challenges some of these assumptions.