Viral infections are normally controlled by T-cell immunity and are a cause of significant morbidity and mortality during the period of immune recovery after hematopoietic stem cell transplantation (HSCT). Risk for infection is impacted by the degree of tissue mismatch between donor and recipient and the immune status of the donor, including the degree and length of immunosuppression following transplantation. Reactivation of latent viruses such as cytomegalovirus (CMV), Epstein-Barr virus (EBV), and Human Herpesvirus 6B (HHV6) are common and often cause symptomatic disease. Reactivations of the polyomaviruses BK virus and JC virus are also common and frequently cause renal disease including hemorrhagic cystitis and less commonly neurologic disease (pervasive multifocal leukoencephalopathy). Respiratory viruses such as adenovirus and human parainfluenza also frequently cause infection. Antiviral pharmacologic agents are only effective against some of these viruses; their use is costly, and associated with significant toxicities and the outgrowth of drug-resistant mutants. As delay in recovery of virus-specific cellular immune response is clearly associated with viral reactivation and disease in these patients, cellular immunotherapy to restore viral-specific immunity is an attractive option that has already been successfully used to target several of these viruses.