Nonalcoholic fatty liver disease (NAFLD) is being increasingly recognized as a common liver disorder that represents the hepatic manifestation of the metabolic syndrome, a variably defined aggregate of disorders related to obesity, insulin resistance, type II diabetes, hypertension and hyperlipidemia. Nonalcoholic steatohepatitis (NASH) is the progressive form of liver injury that carries a risk for progressive fibrosis, cirrhosis, and end-stage liver disease. Hepatocellular carcinoma (HCC) is a documented complication in an as yet unknown percentage of cases of NASH cirrhosis. The diagnosis of nonalcoholic steatohepatitis requires histopathologic evaluation because the lesions of parenchymal injury and fibrosis cannot be detected by imaging studies or laboratory tests. This article will briefly discuss prevalence studies and the pathophysiology of NAFLD and focus on current discussions related to the specific lesions in the pathology of NASH, including the challenges of pediatric NASH and NASH-related cirrhosis.