The authors summarize and update the most recent knowledge in the field of prevalence, natural history and incidence of Non Alcoholic Fatty Liver Disease (NAFLD) and Non Alcoholic Steatohepatitis (NASH). These novel diseases, firstly recognized at the beginning of the second millennium, arose suddenly to the attention of the clinicians, because they are the hepatic expression of the "so-called" metabolic syndrome. Due to the epidemic burden of obesity, diabetes, and metabolic diseases, NAFLD and NASH will become soon probably the most common hepatic disease worldwide, and they surely will keep busy our future young hepatologists.