Killer Ig-like receptors (KIRs) are surface inhibitory receptors specific for allelic forms of human leukocyte antigen (HLA) class I molecules, which are expressed by natural killer (NK) cells and a subset of T lymphocytes. Upon engagement with HLA class I molecules, KIRs block NK cell activation and function. Cells lacking HLA class I molecules are promptly killed by NK cells because of the predominant effect of several activating NK receptors. The NK-mediated killing of these cells might represent an important defence mechanism, antagonizing spreading of pathogens and tumours. Evidence has been accumulated that KIR-encoding genes have evolved and diversified rapidly in primates and in humans. Similar to HLA loci, KIR sequences are highly polymorphic and, moreover, KIR haplotypes greatly vary in the number of the type of genes they contain. KIR gene expression is regulated by mechanisms of DNA methylation. As recently shown, the HLA class I regulated control of NK cell function can be exploited in an allogeneic bone marrow transplantation setting to eradicate acute myeloid leukaemias.