Natural killer (NK) cells are effectors of the innate immune system and recognize cells transformed by viruses or neoplasia. Their response to "missing self" signals was described 3 decades ago, but the recent discovery of a panoply of activating receptors has made it clear that NK cell reactivity arises from a combination of inhibitory and activating signals. Successful clinical exploitation of NK cell reactivity was demonstrated in allogeneic transplantation for acute myelogenous leukemia from HLA-haploidentical donors when matched donors were not available. Multiple clinical studies have since attempted to use NK reactivity in the setting of both HLA-matched and -mismatched transplantation, with varying results. This review summarizes the heterogeneous clinical results and explains them based on a succinct description of NK cell biology.