Nonalcoholic fatty liver disease (NAFLD) is recognized as the most common cause of chronic liver disease in western countries. NAFLD is etiologically associated with systemic and hepatic insulin resistance and is considered by many as the hepatic manifestation of the metabolic syndrome. NAFLD has a wide histological spectrum ranging from 'simple' steatosis to nonalcoholic steatohepatitis (NASH), which may progress to cirrhosis. Hepatocellular carcinoma may occur in NASH-related cirrhosis. The diagnosis of NAFLD/NASH is based on clinico-pathological criteria. Currently available noninvasive tests for the diagnosis of NASH lack specificity and sensitivity, so liver biopsy, despite its limitations, still remains the 'golden standard' for confirming or excluding NASH in a patient with chronically-elevated liver enzymes and image-detected steatosis. This review examines the currently used criteria for the histopathological diagnosis of NAFLD/NASH in adults and children and the relevant histological scoring systems.