This review covers the diagnosis and epidemiology of nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) in children and adolescents. Pediatric NAFLD remains a clinicopathologic diagnosis requiring direct demonstration of liver steatosis and the exclusion of other causes of fatty liver and/or hepatitis. NAFLD is the most common cause of chronic liver disease in children. The number of children with NAFLD presents a major public health crisis. Age, sex, race, ethnicity, and body habitus all influence the risk for NAFLD. The epidemiology of pediatric NAFLD should inform future attempts to develop rigorously evaluated screening protocols. Moreover, these data should guide efforts to delineate the pathophysiology in children and adolescents. Our future ability to prevent and treat pediatric NAFLD is dependent upon such work.