Nonalcoholic fatty liver disease (NAFLD) is a chronic liver disease that has been shown to progress to cirrhosis and hepatocellular carcinoma. This article reviews the prevalence of NAFLD and the factors associated this disorder, and with the more advanced stages of NAFLD, including nonalcoholic steatohepatitis (NASH) and fibrosis. In the general population, the estimated prevalence ranges from 3% to 24%, with most estimates in the 6% to 14% range. NAFLD is extremely common among patients undergoing bariatric surgery, ranging from 84% to 96%. In these patients, 25% to 55% have NASH, 34% to 47% have fibrosis, and 2% to 12% have bridging fibrosis or cirrhosis. NAFLD appears to be most strongly associated with obesity, and insulin resistance states including diabetes and with other features of the metabolic syndrome, such as high triglycerides and low HDL. It appears to be more common in men, and it increases with increasing age and after menopause. Some data suggest that Mexican Americans are more likely to have NAFLD and blacks are less likely compared with non-Hispanic whites. More advanced stages of NAFLD are associated with older age, higher body mass index, diabetes, hypertension, high triglycerides, and/or insulin resistance. An AST/ALT ratio greater >1 may also indicate more severe disease. Although hepatocellular carcinoma can occur in the setting of NAFLD, the risk factors for hepatocellular carcinoma in the setting of NAFLD have not been established. More prospective studies are needed to determine the true risk factors for the development and progression of NAFLD to help identify patients at highest risk who might benefit from treatment trials.