Uterine natural killer (uNK) cells are the most abundant leukocytes in preimplantation endometrium and early pregnancy decidua. Maternal uNK cells are adjacent to, and have the ability to interact directly with, fetal trophoblasts. uNK cells can secrete an array of cytokines that are important in angiogenesis and thus placental development and the establishment of pregnancy. Increased numbers of uNK cells have been associated with reproductive failure. The number of preimplantation uNK cells has been reduced with prednisolone. However, despite these exciting advances in understanding of the uNK cells, considerably more work needs to be done to establish a specific role for uNK cells and to use uNK cells as a test of malfunctioning endometrium and the basis for future treatment for reproductive failure.