Natural killer (NK) cells, a small subset of peripheral blood lymphocytes, mediate a variety of functions that are important in human health and disease. In addition to their role in control of metastasis and infections, NK cells participate in immunoregulation, haematopoiesis, reproduction and neuroendocrine interactions. Defective or absent NK cell activity is associated with a spectrum of human diseases. Immunotherapy to augment or replace NK cell activity is becoming available. Therapeutic modification of NK cell activity is likely to become more widely used as our knowledge of the relevance of NK cells to health and disease states increases in the future.