Adefovir is active against hepadnaviruses in vitro, including all common forms of lamivudine-resistant HBV (rtL180M, rtM204I, rtM204V, rtL180M/ rtM204V), famciclovir-associated mutations (rtV173L, rtP177L, rtL180M, rtT184S or rtV207I) and hepatitis B immunoglobulin escape mutations (rtT128N and rtW153Q), and in in vivo animal models of hepadnavirus replication.