Natural killer (NK) cells were originally defined by their ability to lyse tumor cells or virus-infected cells and identified as one type of effector cells of the non-antigen specific innate resistance. However, many recent studies have widened the interpretation of the role of NK cells in immunity and shown that NK cells have important regulatory roles in innate resistance, antigen-specific adaptive immunity, and, possibly, in hematopoiesis. These functions of NK cells more than on their cytotoxic activity, are probably dependent on their ability to produce lymphokines, particularly interferon-gamma (IFN-gamma). NK cells are important for antigen-independent activation of phagocytic cells early in infection and for favoring the development of antigen-specific T helper cells type I, producing IFN-gamma and IL-2. A role for NK cells in suppression of hematopoiesis and in induction of septic shock may represent a pathological exaggeration of the physiologic functions of NK cells in innate resistance.