During the past 10 years nonalcoholic fatty liver disease (NAFLD) has moved more and more into the focus of attention of clinical research. The term NAFLD comprises the simple nonalcoholic steatosis or fatty liver (NAFL) and nonalcoholic steatohepatitis (NASH). As the incidence of NAFLD has dramatically increased, it is expected that during the coming years NASH in particular will become the main reason for liver transplantation in industrialized countries. In addition, it has become evident that scirrhotic liver fibrosis can develop directly from NAFL without going through the stage of NASH. Finally, there is accumulating evidence that both NAFL and NASH are strongly associated with an increased risk of type2 diabetes and cardiovascular diseases and on the other hand that NAFL and NASH are complications of diabetes. This article addresses the current knowledge about the prevalence, diagnosis, risk prediction and treatment of NAFLD.