Although the optimal donor for allogeneic hematopoietic stem cell transplantation is a human leukocyte antigen (HLA)-matched sibling, 75% of patients do not have a match and alternatives are matched unrelated volunteers, unrelated umbilical cord blood units, and full HLA-haplotype-mismatched family members. This review will focus on the open issues of allogeneic hematopoietic transplantation and on the benefits of natural killer (NK) cell alloreactivity and its underlying mechanisms. Donor-versus-recipient NK cell alloreactivity derives from a mismatch between inhibitory receptors for self major histocompatibility complex (MHC) class I molecules on donor NK clones and the MHC class I ligands on recipient cells. These NK clones sense the missing expression of the self MHC class I allele on the allogeneic targets ("missing self") and mediate alloreactions. Here, we review the translation of NK cell allorecognition into the clinical practice of allogeneic hematopoietic transplantation and discuss how it has opened innovative perspectives in the cure of leukemia.