Medical advances such as allogeneic transplantation can expose patients to periods of marked immunosuppression, during which viral infections are an important cause of morbidity and mortality. Control of infection will depend ultimately on the restoration of adequate antiviral immunity, and cellular immunotherapy is an attractive approach to improving immune protection. Developments in basic immunology have led to a greater understanding of the nature of protective immunity in immunocompetent donors, and this knowledge is now being used to direct immunotherapeutic protocols. Moreover, immunological techniques that have recently been developed as research tools, such as peptide–HLA tetramers and cytokine-secretion assays, have potential application for clinical use in this setting.