Nonalcoholic fatty liver disease (NAFLD) is defined as fatty infiltration of the liver exceeding 5% to 10% by weight. It is a spectrum of disorders ranging from simple fatty liver (steatosis without liver injury), nonalcoholic steatohepatitis (steatosis with inflammation), and fibrosis/cirrhosis that resembles alcohol-induced liver disease but which develops in individuals who are not heavy drinkers. NAFLD is likely the most common cause of chronic liver disease in many countries. NAFLD may also potentiate liver damage induced by other agents, such as alcohol, industrial toxins and hepatatrophic viruses. The lack of specific and sensitive noninvasive tests for NAFLD limits reliable detection of the disease. It is often diagnosed on a presumptive basis when liver enzyme elevations are noted in overweight or obese individuals without identifiable etiology for liver disease, or when imaging studies suggest hepatic steatosis. NAFLD is now considered to be a component of the insulin resistance syndrome (metabolic syndrome X). Controversy exists relative to optimal recognition, diagnosis and management of these conditions, and treatment recommendations are evolving.