Nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) are the most common underlying causes of chronic liver injury. They are associated with a wide spectrum of hepatic disorders including basic steatosis, steatohepatitis, and cirrhosis. The molecular and cellular mechanisms underlying hepatic injury in NAFLD and NASH are still unknown. This review describes the roles of oxidative stress and inflammatory responses in the pathogenesis of NAFLD and its progression to NASH.