The risk of infection after allogeneic stem cell transplantation is determined by the underlying disease, the intensity of previous treatments and complications that may have occurred during that time, but above all, the risk of infection is determined by the selected transplantation modality (e.g. HLA-match between the stem cell donor and recipient, T cell depletion of the graft, and others). In comparison with patients treated with high-dose chemotherapy and autologous stem cell transplantation, patients undergoing allogeneic stem cell transplantation are at a much higher risk of infection even after hematopoietic reconstitution, due to the delayed recovery of T and B cell functions. The rate at which immune function recovers after hematopoietic reconstitution greatly influences the incidence and type of post-transplant infectious complications. Infection-associated mortality, for example, is significantly higher following engraftment than during the short neutropenic period that immediately follows transplantation.