In primates, including women, and in rodents, Natural Killer lymphocytes (NK cells) have a unique relationship with the decidualizing uterus. Implantation sites from genetically-modified and transplanted mice have proven useful models for understanding potential mechanisms involved in recruitment, activation and functions of human CD56 bright uterine (u) NK cells. Key findings are reviewed. In mice, uNK precursor cells are recruited from secondary lymphoid tissues and are activated coincident with their uterine arrival. UNK cells proliferate, produce cytokines (interferon gamma (IFN-γ) and interleukins (IL)-18 and IL-27), and terminally differentiate into granulated lymphocytes. Many uNK cells proliferate within the myometrium at each implantation site forming a structure, the mesometrial lymphoid aggregate of pregnancy (MLAp) that surrounds blood vessels servicing each placenta. Post-mitotic uNK cells are abundant within decidua basalis; frequently (> 25%) associating with spiral arteries, intramurally and intralumenally. From midgestation, uNK cell numbers decline. Studies of implantation sites in mice lacking uNK cells, IFN-γ, components of IFN-γ-induction and-signalling pathways or IFN-γ-regulated genes, indicate that uNK cell-derived IFN-γ is essential in triggering pregnancy-induced spiral artery modification. Decidual maintenance and uNK cell death are additional effects of uNK-cell-derived IFN-γ. Thus, during the first half of gestation, UNK cells contribute to and sustain important changes in the maternal placental bed.