Natural killer (NK) cells are important effectors for the lysis of both neoplastic and virus‐infected cells. Lectin‐like receptors on human NK cells, such as NKR‐PIA and CD94, bind to target cell carbohydrate ligands and initiate the lytic process. In addition, P58 and P70 bind to major histocompatibility class I antigens on targets and mediate negative signals. Models using NK cell‐deficient mice have proven useful in elaborating the role of NK cells in the immune defence against multiple viral agents. In addition, studies in humans have suggested a vital role of NK cells in the host defence against human immunodeficiency virus, herpesviruses, hepatitis B and C and other viruses. Several genetic disorders, chronic illnesses and infections have been associated with decreased NK function.