Nonalcoholic fatty liver (NAFL) is increasingly recognized as an important and common public health problem that can lead to cirrhosis and hepatic failure. Because it is often asymptomatic,many people may not know that they have it. NAFL is closely linked to obesity, which in the United States and other developed countries is becoming more common. Consequently, the proportion of the population affected by NAFL will likely increase. Despite the growing importance of this condition, knowledge of the epidemiology of NAFL is limited by the lack of an accurate,noninvasive measure for use in screening of the general population. This article reviews information available from studies with relatively unselected samples with regard to prevalence, demographics,and risk factors for NAFL.