Abstract Occult hepatitis B virus (HBV) infection is generally defined as the detection of HBV-DNA in the serum or liver tissue of patients who test negative for hepatitis B surface antigen. In most cases, occult HBV infection is related to low level HBV infection with subdetectable levels of HBsAg and not infection with HBV variants that cannot express S proteins or produce S proteins with aberrant epitopes that are not detected by conventional serological assays. Prevalence of occult HBV infection is related to the overall prevalence of HBV infection in that country, being more common in persons with prior exposure to HBV. Occult HBV infection has been found in a substantial proportion of patients with cirrhosis and hepatocellular carcinoma but other causes of liver disease are frequently present. Future studies should focus on delineating the pathogenic role of occult HBV infection and the basis for failure to detect circulating hepatitis B surface antigen.