Although the vast majority of heavy drinkers and individuals with obesity, insulin resistance, and the metabolic syndrome have steatosis, only a minority ever develop steatohepatitis, fibrosis, and cirrhosis. Genetic and environmental risk factors for advanced alcoholic liver disease (ALD) and nonalcoholic fatty liver disease (NAFLD) seem likely to include factors that influence the severity of steatosis and oxidative stress, the cytokine milieu, the magnitude of the immune response, and/or the severity of liver fibrosis. For ALD, the dose and pattern of alcohol intake, coffee intake, and dietary and other lifestyle factors leading to obesity are the most important environmental determinants of disease risk. For NAFLD, dietary saturated fat and antioxidant intake, small bowel bacterial overgrowth, and obstructive sleep apnea syndrome may play a role. Family studies and interethnic variations in susceptibility suggest that genetic factors are important in determining disease risk. For ALD, functional polymorphisms in the ADH and ALDH alcohol metabolizing genes play a role in determining susceptibility in Oriental populations. No genetic associations with advanced NAFLD have been replicated in large studies. Preliminary data suggest that polymorphisms in the genes encoding microsomal triglyceride transfer protein, superoxide dismutase 2, the CD14 endotoxin receptor, tumor necrosis factor alpha, transforming growth factor beta, and angiotensinogen may be associated with steatohepatitis or hepatic fibrosis or both.