The hepatitis B virus (HBV), discovered in 1966, infects more than 350 million people worldwide.1 Hepatitis B is a leading cause of chronic hepatitis, cirrhosis, and hepatocellular carcinoma, accounting for 1 million deaths annually. Knowledge of the intricacies of viral infection and of the molecular biology of this fascinating virus has led to the successful development of a vaccine and to treatment sometimes capable of eradicating chronic infection. This review addresses many aspects of HBV infection, including the role of the immune system in determining the outcome of clinical infection, recent developments in molecular studies of the virus, and new . . .