Infection remains an important source of morbidity and mortality in patients who undergo hematopoietic stem cell transplantation (HSCT). In the immune reconstitution period after transplantation, HSCT recipients are most likely to have bacterial or fungal infections. Invasive fungal infections (IFIs) and mycobacterial infections (MBIs) are among the complications of HSCT, with high morbidity and mortality rates. Early diagnosis of both is crucial in order to manipulate the disease and to avoid fulminant outcomes. This chapter reviews the current knowledge on the molecular diagnosis of IFIs and MBIs in HSCT recipients, describing two different polymerase chain reaction (PCR)-based methods, one commercial (qPCR, Roche) and one in-house IS6110-based protocol.