Natural killer (NK) cells are large granular lymphocytes of the innate immune system that exert a potent function against infected and tumor cells. Although NK cells were originally defined by their capacity to lyse target cells and produce interferon-gamma without prior activation, recent studies showed that NK cells also display a potent regulatory function. They are activated or inhibited through the ligation of germline-encoded receptors and are involved in mediating cytotoxicity, producing cytokines and providing costimulation to cells of the adaptive immune system. NK cells play important roles in viral infections, autoimmunity, pregnancy, cancer and bone marrow transplantation, but little is known about the role of NK cells in allergy. Recent developments in the understanding of the role of human NK cells in allergy are overviewed.