This review covers the diagnosis and outcomes of histological subphenotypes of nonalcoholic fatty liver disease (NAFLD) including nonalcoholic steatohepatitis (NASH) in both children and adults. Serious outcomes of NAFLD include cirrhosis, hepatocellular carcinoma, coronary heart disease, and diabetes. The ability to determine differential outcomes by NAFLD subtypes is dependent upon both the adequate assessment of histological subtypes, as well as the appropriate measure of the outcome of interest. Steatohepatitis, which can only be diagnosed by liver biopsy, has a greater risk for both hepatic and systemic complications than simple steatosis. Differences in subphenotypes between children and adults and among children have only recently been described. Careful future attention should be given to determine their influence on clinical outcomes. Enhanced understanding of the epidemiology of NAFLD subphenotypes may help organize collaborative efforts between hepatologists, pathologists, cardiologists, endocrinologists, and primary care physicians to develop clinical management protocols.