Hepatitis B virus (HBV) is the human member in the family Hepadnaviridae . Several bird and rodent hepadnaviruses have been isolated, including duck (1 ), heron (2 ), woodchuck (3 ), ground squirrel (4 ), and Arctic ground squirrel HBVs (5 ). A primate hepadnavirus has recently been isolated from a New World monkey and was designated woolly monkey HBV (6 ). Approximately 5–10% of HBV-infected adults and almost all infected newborns become chronically infected. Chronic carriers of HBV may have an asymptomatic infection for decades and yet still develop liver cirrhosis or hepatocellular carcinoma. More than 350 million chronic carriers of HBV exist worldwide. Despite a vaccine and several efficacious antivirals, current antiviral therapy does not result in viral clearance. The purification of the HBV polymerase (pol) and experimental in vitro assays for pol are useful for the evaluation of antivirals that may inhibit HBV replication.