Fatty liver can be an independent primary disease, but is more often a manifestation of a systemic disease involving the liver. There are many causes of excessive fat accumulation in the liver, and the extent of their effects and the relationship between them are still not fully understood. Clinically, common causes of fatty liver include excessive alcohol consumption, obesity, diabetes mellitus, hyperlipidemia, starvation, high-fat low-protein diet, protein-calorie deficient malnutrition, intravenous high-energy nutritional therapy, small intestine diversion surgery, drugs or industrial toxins such as tetracycline, carbon tetrachloride, yellow phosphorus and amiodarone, hepatitis virus infection (especially hepatitis C virus infection), acute fatty liver in pregnancy, Reye’s syndrome, endocrine metabolic disorders, and hepatitis B. Reye syndrome, endocrine metabolic disorders (e.g., cortisolism, hyperthyroidism, long-course glucocorticoid therapy), and some chronic infections and inflammatory conditions (e.g., inflammatory bowel disease, chronic tuberculosis). The etiological composition ratio, clinical manifestations and prognosis of fatty liver vary at different times, in different countries and populations. For example, during the three-year natural disaster period, malnutrition fatty liver was particularly common among rural residents in China. In recent years, with the improvement of economic level and the increase of fatty foods and alcohol consumption, obesity, diabetes mellitus and hyperlipidemia have become the main causes of fatty liver in developed regions such as Shanghai, while fatty liver in northern and minority regions is mainly caused by alcoholism. Sometimes, several causes can coexist in a particular patient, such as alcohol consumption, hepatitis virus infection, excess weight and medications.