Non-alcoholic fatty liver disease (NAFLD) is a broad term that includes liver diseases characterized by abnormal hepatocellular accumulations of lipid that cannot be related to alcohol abuse. It may be found in both adults and children, particularly those who are obese or have insulin resistance. Steatohepatitis is a specific pattern of injury within the spectrum of NAFLD and this pattern is associated with fibrotic progression and cirrhosis. In addition to steatohepatitis, a distinct form of fibrotic fatty liver disease exists in children. There have been a number of recent advances in the characterization of histologic changes in NAFLD. In light of these recent reports, this study will: (1) review the histologic features of steatosis and nonalcoholic steatohepatitis in adults; (2) review the variation of histologic patterns of pediatric fatty liver disease; and (3) discuss the validity and use of the nonalcoholic fatty liver disease Activity Score.