Hepatitis B virus is a member of the family Hepadnaviridae and is responsible for causing acute and chronic liver infections in humans. Current estimates of the total number of chronic carriers of the virus are put at 350 million worldwide. These individuals run the risk of developing cirrhosis of the liver and hepatocellular carcinoma in later life. Antiviral treatment with interferon or nucleos(t)ide analogs offers the only means of interrupting this process. This article reviews the current level of knowledge relating to this important human pathogen and deals in turn with the taxonomy of the virus, its structure and genome organization, life cycle and replication strategy, as well as current preventative approaches and therapeutic interventions.