Hepatitis B virus (HBV) is a DNA-virus that spreads through sexual contact, perinatally or through contact with blood. Around the world there are aproximately two billion people who, at some point of their life, have been infected with HBV and have developed antibodies against HBV, but their hepatitis B surface antigen (HbsAg) is not detectable. These people are at risk of HBV reactivation (HBVr), which may be spontaneous or iatrogenic. HBVr is defined as HbsAg seroconversion if undetectable HBV DNA becomes detectable or in the case of an over 2 log10 increase in HBV DNA levels from baseline level. One of the growing iatrogenic causes of reactivation is immunosuppression through immunosuppressive medications and therapies. Therefore, all patients undergoing immunosuppression should be tested for HBV infection by detecting HbsAg, HBV surface antibody (HbsAb) and HBV core antibody (HbcAb). If any of the given markers is positive, HBV DNA detection should follow. Everybody with detectable HBV DNA levels should be treated with antiviral agents before immunosuppression. If HBV DNA is not detectable then the risk of HBVr should be evaluated according to the length of treatment and the immunosupressive agent used. It is important to remember that any combination of HBV markers is possible and HBV infection is always dangerous.