Viral infections are a leading cause of posttransplantation morbidity and mortality. A number of recent developments have altered our understanding and management of these disorders. The pathogenetic roles of several viruses, including human herpesviruses 6 and 8, have been newly established. Molecular-based diagnostic tests now make more rapid diagnosis possible. The licensing of new potent antiviral agents offers a wider choice of drugs for viral prophylaxis and treatment. The use of more potent immunosuppressive agents is responsible in part for the increasing incidence of some viral infections, but this varies among drugs, and individual viruses differ in their sensitivity to immunosuppressive agents. This review summarizes the natural history, diagnosis, prevention, and treatment of many common viral infections after renal transplantation.