Nonalcoholic fatty liver disease is defined as the presence of hepatic fat accumulation, proven by histological analysis or imaging techniques, in the absence of other causes of steatosis, and includes two entities: nonalcoholic fatty liver and nonalcoholic steatohepatitis with possible progression to fibrosis and cirrhosis. The estimated global prevalence of the disease is around 25% and it is increasing. The pathogenesis of the disease is complex, and is usually explained by the "two hits" hypothesis, the first of which is fat accumulation and the other damage to hepatocytes and inflammation. Additional mechanisms are involved in the development of fibrosis. Nonalcoholic fatty liver is associated with insulin resistance, but other different cytokines and adipokines are also involved in the pathogenesis of the disease. Clinical feature is nonspecific, and in diagnosing laboratory findings, imaging techniques, and liver biopsy are used. Liver biopsy still remains the gold standard for diagnosis and assessment of disease progression. The treatment should be tailored individually, and the effect can be achieved by body mass reduction, increase in physical activity, pharmacologic therapies with pioglitazone and vitamin E, and bariatric surgery. Nonalcoholic fatty liver disease can lead to death, caused by the liver disease alone, but more frequently causes of death are associated with cardiovascular diseases.