Cytomegalovirus (CMV) and Epstein-Barr virus (EBV) infections following allogeneic hematopoietic stem cell transplantation (HSCT) are a major cause of morbidity and mortality. Early clinical trials demonstrate that adoptive transfer of donor-derived virus-specific T cells to restore virus-specific immunity is an effective strategy to control CMV and EBV infection after HSCT, conferring protection in 70%-90% of patients. The field has evolved rapidly to develop solutions to some of the manufacturing challenges identified in early clinical studies, such as prolonged in vitro culture, optimization of the purity of the virus-specific T cell product, the potential limitations of targeting a single viral antigen, and how to manage the patient with a virus-naive donor. This Review both discusses the seminal early studies and explores cutting-edge novel technologies that broaden the feasibility of and the scope for delivering virus-specific T cells to patients after HSCT.