Occult hepatitis B virus (HBV) infection has been identified as a separate entity since nucleic acid amplification assays of enhanced sensitivity became employed for detection of hepadnaviral genomes and their replicative intermediates. This silent form of HBV carriage is a consequence of a low-level, persistent virus replication progressing in the liver and/or in the immune system, and is defined as the existence of HBV DNA in serum, lymphoid cells and/or hepatic tissue in the absence of serum HBV surface antigen measured by standard clinical laboratory assays. Much of the understanding of the natural course, virological and immunological properties, requirements for transmission, and potential pathological and epidemiological ramifications of occult HBV infection is owed to research in the woodchuck model of hepatitis B. This review summarizes findings in the woodchuck experimental system which were central to the identification, delineation of characteristics, and recognition of long-term consequences of occult hepadnaviral persistence.  It also outlines features differentiating two forms of occult hepadnaviral infection, the existence of which became evident through investigations in the woodchuck model, i.e., secondary occult infection (SOI), continuing after resolution of hepatitis or serologically apparent but asymptomatic infection and primary occult infection (POI), which is serologically undetectable and restricted to cells of the lymphatic (immune) system from its initiation, although with time it may also engage the liver.