Viruses are among the most common causes of opportunistic infection after transplantation and the most important. The risk for viral infection is a function of the specific virus encountered, the intensity of immune suppression used to prevent graft rejection, and other host factors governing susceptibility. Viral infection, both symptomatic and asymptomatic, causes the "direct effects" of invasive disease and "indirect effects," including immune suppression predisposing to other opportunistic infections and oncogenesis. Rapid and sensitive microbiologic assays for many of the common viruses after transplantation have replaced, for the most part, serologic testing and in vitro cultures for the diagnosis of infection. Furthermore, quantitative molecular tests allow the individualization of antiviral therapies for prevention and treatment of infection. This advance is most prominent in the management of cytomegalovirus, Epstein-Barr, hepatitis B, and hepatitis C viruses. Diagnostic advances have not been accompanied by the development of specific and nontoxic anti-viral agents or effective antiviral vaccines. Vaccines, where available, should be given to patients as early as possible and well in advance of transplantation to optimize the immune response. Studies of viral latency, reactivation, and the cellular effects of viral infection will provide clues for future strategies in prevention and treatment of viral infections.