Abstract: The present invention provides the tertiary structure of a Hepatitis B Virus (HBV) polymerase reverse transcriptase (rt) domain from which a variant HBV polymerase mutations associated with resistance to or having reduced sensitivity to an anti-viral drug have been mapped. The present invention further provides methods of identifying, designing and/or modifying agents capable of modulating the functional activity of the HBV polymerase based on the atomic co-ordinates provided by the tertiary structure. The present invention still further provides a method of modulating HBV polymerase functional activity and agents useful for same. The agents identified in accordance with the method of the present invention are particularly useful inter alia in the treatment and/or prophylaxis of infection by an HBV resistant to or exhibiting reduced sensitivity to an anti-viral drug. The agents may also have utility as diagnostic agents such as to distinguish between resistance mutations.