Organ transplantation is a lifesaving procedure for many patients. To prevent rejection or graft-versus-host disease, recipients require long-term immunosuppression. In patients who have ever been exposed to hepatitis B, it is possible for reactivation to occur; this includes patients who are anti–hepatitis B core antibody-positive only or both anti–hepatitis B core antibody-positive and hepatitis B surface antibody-positive. The susceptibility to this varies with the nature of the transplant. Hepatitis B can be transmitted from donor to recipient. It is important to assess the hepatitis B status and formulate a strategy to prevent transmission and prevent reactivation.