Viral infections represent one of the main causes of morbidity and mortality following Hematopoietic Stem Cell Transplantation. Anti-viral treatment failure may be explained by absence of specific immune reconstitution. In the 1990s, anti-viral immunotherapy initially consisting in total donor lymphocyte infusion presented efficiency but was often associated with adverse effects. Specific antiviral immunotherapy emerged and relied on isolation of mono or multi-virus donor-derived-specific T cells with or without in vitro expansion. During the last ten years, such an adoptive transfer has been proved feasible, and helpful in specific anti-viral immune reconstitution, and rarely associated with advers events. Two main evolutions contributed to allow a good reactivity to propose immunotherapy in case of anti-viral treatment failure: development of allogeneic CTL banks and improvement of CTL isolation methods using immunomagnetic technology which presents the advantage to be fast.