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Non-alcoholic fatty liver disease (NAFLD) is rapidly becoming the most common liver disease worldwide. The prevalence of NAFLD in the general population of Western countries is 20-30%. About 2-3% of the general population is estimated to have non-alcoholic steatohepatitis (NASH), which may progress to liver cirrhosis and hepatocarcinoma. As a rule, the prevalence of NAFLD is higher in males and increases with increasing age, and it is influenced by the diagnostic method and the characteristics of the population, especially lifestyle habits. Population-based studies provide better estimates of the prevalence of NAFLD as compared to autoptic and clinical studies, but few such studies have been performed to date. The diagnosis of NAFLD in population studies is usually obtained by ultrasonography, which is known to underestimate the prevalence of fatty liver. The Dallas Heart Study and the Dionysos Study reported that 30% of the adults in the USA and 25% in Italy have NAFLD. In these studies, 79% and 55% of patients with NAFLD had normal aminotransferase levels, showing that liver enzymes are not surrogate markers of NAFLD in the general population. Noninvasive markers such as the fatty liver index obtained from the Dionysos Study may be useful to screen for NAFLD in the general population. The most important risk factors for NAFLD are male gender, age, obesity, insulin resistance and the cardiometabolic alterations that define the metabolic syndrome. The prevalence of NAFLD is 80-90% in obese adults, 30-50% in patients with diabetes and up to 90% in patients with hyperlipidemia. The prevalence of NAFLD among children is 3-10%, rising up to 40-70% among obese children. Moreover, pediatric NAFLD increased from about 3% a decade ago to 5% today, with a male-to-female ratio of 2:1. The incidence and natural history of NAFLD are still not well defined, but it is recognized that the majority of individuals with NAFLD do not develop NASH. The incidence of NAFLD is probably increasing in Western countries, strictly linked to lifestyle habits.