Non-alcoholic fatty liver disease (NAFLD) is a clinicopathologic entity increasingly recognized as a major health burden in developed countries. It includes a spectrum of liver damage ranging from simple steatosis to nonalcoholic steatohepatitis (NASH), advanced fibrosis, and rarely, progression to cirrhosis. Recent studies emphasize the role of insulin resistance, oxidative stress and subsequent lipid peroxidation, proinflammatory cytokines, adipokines and mitochondrial dysfunction in the development and progression of NAFLD. Furthermore, accumulating evidence supports an association between NAFLD and metabolic syndrome. Although the data are mainly epidemiological, the pathogenesis of NAFLD and metabolic syndrome seems to have common pathophysiological mechanisms, with focus on insulin resistance as a key factor. This review summarizes the current knowledge on the epidemiology, pathophysiology and diagnosis of both NAFLD and metabolic syndrome and the findings that strongly support the association of nonalcoholic fatty liver disease as a possible component in the cluster of metabolic syndrome.