Nonalcoholic fatty liver disease is one of the most common causes of liver disease in the United States, and its prevalence appears to be increasing. The disease shows a range in severity, from simple steatosis, which is usually nonprogressive and benign, to nonalcoholic steatohepatitis (NASH), which is a progressive liver disease that can result in cirrhosis and hepatocellular carcinoma. The Nonalcoholic Steatohepatitis Research Network was established by the NIDDK in response to the lack of clear understanding of the pathogenesis of NASH, its natural history, prognostic features, and treatment. The primary objective of the NASH CRN is to pursue clinical research on adult and pediatric NASH. The network also aims to encourage translational research focusing upon elucidating the pathogenesis that will provide the basis for understanding the natural history and developing means of diagnosis, treatment and clinical management. The NASH CRN conducts numerous observational studies and clinical trials. The Network has enrolled over 3500 adults and children into NAFLD databases to determine associations of demographic, clinical, and laboratory variables with the diagnosis of NASH and its key histological features and to identify predictors of severity of NAFLD. Additionally, the NASH CRN has successfully initiated clinical trials for adults and children, including the Farnesoid X Receptor (FXR) Ligand Obeticholic Acid in NASH Treatment (FLINT) Trial, the Pioglitazone vs. Vitamin E vs. Placebo for the Treatment of Non-Diabetic Patients With Nonalcoholic Steatohepatitis (PIVENS) Trial, the Treatment of Nonalcoholic Fatty Liver Disease in Children (TONIC) trial, and CyNCH (Cysteamine Bitartrate Delayed-Release for the Treatment of Nonalcoholic Fatty Liver Disease (NAFLD) in Children (CyNCh) Trial). More information about these studies is available on the individual study pages listed below.