NK cells are physiologically important in a number of contexts: mediating immunity against viruses, intracellular bacteria and parasites, and in anti-tumour immune responses. Moreover, in addition to these overtly immune protective functions, NK cells also mediate a variety of homeostatic functions, particularly in the regulation of haematopoesis and they may have an important role to play in the maintenance and development of placentation; certainly NK cells are a major component of the lymphocyte population of the decidua. The behaviour of the NK cell in these various situations is regulated by a large number of distinct receptors that transmit positive and negative signals. The balance of these signals determines whether the NK cell does nothing or is activated to proliferate, kill or secrete a wide range of cytokines and chemokines. In this review the structure and function of a number of molecules found on the NK cell surface are discussed, particular emphasis being placed on the molecular details of the recognition of target cell classical class I HLA molecules by Killer cell Immunoglobulin-like Receptors (KIR) and the binding of the non-classical class I molecule HLA-E to the heterodimer formed by the association of CD94 with various members of the NKG2 proteins.