The risk of infection in the transplant patient is determined by two factors: the net state of immunosuppression and the environmental exposures the patient encounters. Those infections that do occur in the transplant patient are strongly modulated by the type, intensity, duration, and sequence of immunosuppressive agents administered. A central role in the pathogenesis of all forms of infection in the transplant patient is played by the immunomodulating viruses, particularly cytomegalovirus. Prevention of infection is far better than treatment; when prevention fails and clinical disease develops, patient and allograft survival are directly related to the speed with which diagnosis is made and specific therapy instituted. In order to prevent disease, both antimicrobial prophylaxis and preemptive therapy are being increasingly employed, particularly to blunt the side effects of intensive antirejection therapy.