Occult hepatitis B virus (HBV) infection is characterized by the presence of HBV DNA in serum and/or in the liver of patients negative for hepatitis B surface antigen (HBsAg). Occult infection may impact in several different clinical contexts including the risk of HBV transmission with transfusion or transplantation, and endogenous viral reactivation. The gold standard test for detection of occult infection is the amplification of HBV DNA. However, the serological assay for the long-lasting antibody response to the highly immunogenic HBV core antigen (anti-HBc) represents a qualified candidate as a surrogate for DNA amplification, or for increasing overall sensitivity when assessing the risk of occult hepatitis in peripheral blood. The risk of occult hepatitis associated with anti-HBc seropositivity has been demonstrated extensively, and the presence of antibody response to HBc can be considered a sentinel marker of occult HBV infection.