Natural killer (NK) cells have held great promise for the immunotherapy of cancer for more than 3 decades. However, to date only modest clinical success has been achieved manipulating the NK cell compartment in patients with malignant disease. Progress in the field of NK cell receptors has revolutionized our concept of how NK cells selectively recognize and lyse tumor and virally infected cells while sparing normal cells. Major families of cell surface receptors that inhibit and activate NK cells to lyse target cells have been characterized, including killer cell immunoglobulinlike receptors (KIRs), C-type lectins, and natural cytotoxicity receptors (NCRs). Further, identification of NK receptor ligands and their expression on normal and transformed cells completes the information needed to begin development of rational clinical approaches to manipulating receptor/ligand interactions for clinical benefit. Indeed, clinical data suggest that mismatch of NK receptors and ligands during allogeneic bone marrow transplantation may be used to prevent leukemia relapse. Here, we review how NK cell receptors control natural cytotoxicity and novel approaches to manipulating NK receptor-ligand interactions for the potential benefit of patients with cancer.