Natural Killer (NK) cells are important effector cells in the early control of infected, malignant and 'non-self' cells. Various receptor families are involved in enabling NK cells to detect and efficiently eliminate these target cells. The Killer-cell Immunoglobulin-like Receptor (KIR) family are a set of receptors that are very polymorphic with regards to gene content, expression level and expression pattern. KIRs are responsible for the induction of a NK cell alloreactive response through their interaction with HLA class I molecules. The role of NK cells in Haematopoietic Stem Cell Transplantation (HSCT) has been studied for many years and induction of antileukaemic responses by donor NK cells have been reported. Conflicting data still exist on the exact circumstances in which the KIR repertoire affects and influences clinical outcome after HSCT. More large-scale studies are needed on well-defined cohorts to unravel the mechanism of action of the NK cell-mediated alloresponse in an HSCT setting.