Natural killer (NK) cells have been known for a long time to be a very important component of the innate immune system. However, it is only during the last 10 years that knowledge of their receptors has emerged. Described in the present review are those receptor families killer inhibitory receptor (KIR) (belonging to the immunoglobulin superfamily), and killer lectin like receptor (KLR) CD94/NKG2, that both use HLA as a ligand and have inhibiting and activating types of receptors, and natural cytotoxic receptors (NCR) which do not associate with HLA. Association of the receptor gives rise to either an inhibiting or activating signal leading to either failure or success in lysing a target cell. The KIR receptors are very polymorphic both in the number of genes expressed in an individual and the alleles present for a gene. They would appear to have had a rapid evolution compared to the CD94/NKG2 receptors. The roles that NK cells and their receptors have with various facets of transplantation, disease, pregnancy and control of virus infection in humans are described.