NK cells are the important cells of the immune system derived from stem cells in the marrow. Their physiology is tightly regulated to control proliferation, cytotoxicity and cytokine production. In cancer, NK cells may be abnormal due to the cancer itself or possibly related to its therapy. The finding of class I recognizing inhibitory receptors may play a role in stem cell transplant rejection, immune surveillance and cancer immunotherapy. NK cells should no longer be thought of as direct cytotoxic killers alone, as they clearly play a critical role in cytokine production which may be important to control cancer and infection. Understanding NK cell function and homing may lead to novel therapeutic strategies for the treatment of human disease.