Nonalcoholic fatty liver disease (NAFLD) is fast becoming the most common chronic liver condition in many parts of the world. It is a heterogeneous disease encompassing a broad spectrum of histologic states characterized universally by macrovesicular hepatic steatosis. NAFLD is now recognized as the hepatic manifestation of the metabolic syndrome and is a major cause of liver-related morbidity and mortality. The prevalence of the disease is increasing, tied closely to the increased prevalence of insulin resistance in industrialized countries. Risk factors for NAFLD include obesity, diabetes, insulin resistance, and hypertriglyceridemia. The clinical course of NAFLD depends upon the histologic subtype. Patients with simple hepatic steatosis generally are thought to have a benign long-term prognosis. However, nonalcoholic steatohepatitis can progress to cirrhosis and may have a similar prognosis as cirrhosis from other liver disease with progression to end stage liver disease and hepatocellular carcinoma. The decision to obtain a biopsy is determined by weighing the risks of the biopsy against the information obtained from the biopsy. No standardized therapeutic approach exists although many promising modalities are under investigation. This paper presents an overview of the incidence and prevalence of disease, diagnosis, histologic spectrum, natural history, pathogenesis, clinical course, and current treatment of nonalcoholic fatty liver disease.