Chronic infection by hepatitis B virus (HBV) can result in cirrhosis, hepatic failure, and hepatocellular carcinoma (HCC) if left untreated. The extent of liver injury and the risk of HCC closely correlate with active viral replication, as indicated by high levels of circulating HBV DNA. Five oral antiviral agents, each targeting the viral DNA polymerase, are currently available in the United States to inhibit viral replication. Their effective use requires proper monitoring of drug resistance development. The current clinical guidelines recommend measuring "total" circulating HBV DNA to evaluate the efficacy of treatment and to serve as an indirect measure of drugresistant mutant viruses. This may result in delayed identification of drug-resistant mutants that can cause hepatic injuries and decompensation. This review discusses the virologic mechanisms of drug resistance, the need and strategies for early detection of mutant viruses, and the recent studies on the prevention of drug resistance.