Occult (silent, inapparent) hepatitis B infection is defined as the presence of HBV-DNA where hepatitis B virus (HBV) surface antigen (HBsAg) is absent, outside of acute hepatitis window period. HBV-DNA positivity alone can be detected not only in cryptogenic hepatitis, hepatocellular carcinoma, recovered HBV infections and hepatitis C virus infections, but also in seronegative populations without symptomatic liver diseases. The mechanism, clinical outcome and risk of transmission of occult hepatitis B is not yet clearly defined. Occult hepatitis B can be diagnosed by using sensitive nucleic acid tests. In this review article, the importance of occult hepatitis B infection has been discussed with a focus on pathogenesis, clinical significance and diagnostic laboratory methods.