Nonalcoholic fatty liver disease (NAFLD) is now recognized as both an important component of the metabolic syndrome and the most prevalent liver disease in the United States. Although the mechanisms for development of steatosis and chronic liver injury in NAFLD remain unclear, recent investigations have indicated that overactivation of c-Jun N-terminal kinase (JNK) is crucial to this process. These findings, together with evidence for the involvement of JNK signaling in other manifestations of the metabolic syndrome such as obesity and insulin resistance, have suggested that JNK could be a novel therapeutic target in this disorder. This review details findings that JNK mediates lipid accumulation and cell injury in fatty liver disease and discusses the possible cellular mechanisms of JNK actions.