Infection in marrow transplant recipients is determined primarily by the evolving immunologic milieu of each patient. Profound neutropenia and disruption of anatomic barriers are the most important risk factors for bacterial and fungal infections in the initial period after transplant. After this period, the occurrences of acute and then chronic graft-versus-host disease (GVHD) are the most important influences on the risk of infection. Major infections after the period of initial engraftment include viral infections (especially cytomegalovirus), fungal infections (due to Aspergillus and Candida), and rarely protozoal infections. GVHD appears to increase both the incidence and severity of cytomegalovirus infection. Bacterial infections also continue to occur, due predominantly to coagulase-negative Staphylococcus, as in the neutropenic period. Patients with chronic GVHD have continued abnormalities of host defenses, which may be further suppressed by treatment for GVHD. Major efforts have been directed toward preventing infection. In the neutropenic period, these include use of the protective environment, which has also been associated with a lower incidence of acute GVHD among patients who received transplants for aplastic anemia. The use of seronegative blood products is highly effective in preventing primary cytomegalovirus infection among seronegative patients. Among patients being treated for chronic GVHD, trimethoprim/sulfamethoxazole prophylaxis has been associated with a lower risk of infection.