Infectious complications continue to be a leading cause of morbidity and mortality after stem cell transplantation (SCT). The most important determinants for risk and outcomes are the stem cell source and the time after transplantation that the infection occurs. More recent developments in preparative regimens and graft manipulations, as well as the control of some important infectious complications after SCT by the introduction of effective prophylaxis and preemptive treatment strategies, have had a great impact on the timing and the epidemiology of infections posttransplantation. Better control of infectious complications posttransplantation has resulted in a reduced transplant-related mortality.