© 2020 Tohoku University Medical Press. The uterus is an organ for raising the fetus, and its lumen is lined by the endometrium. The endometrium is an important site for the implantation and maturation of fertilized eggs. The endometrium undergoes repetitive proliferation, maturation (decidualization), and exfoliation changes every menstrual cycle. At the same time, the number and type of endometrial immunocompetent cells vary during the menstrual cycle. At the implantation stage, the immunocompetent cells occupy approximately half of the endometrial cells. Immunocompetent cells normally eliminate pathogenic microorganisms to protect the body; however, they also promote immune tolerance to accept the fetus during pregnancy. The immunocompetent cells in the uterus can perform both these functions. With the establishment of pregnancy, stimuli from the trophoblast (placenta) and fetus can also change the immune environment of the uterus, and pregnancy can be maintained only when the immune system is well adapted to the stimuli of some hormones and the fetus. Immunity for the establishment of pregnancy is not simple because multiple immunocompetent cells are involved in establishing and maintaining pregnancy. To understand the immune mechanisms associated with the establishment of pregnancy, we have to learn about each immune cell. This review, therefore, discusses the roles and distribution of the immunocompetent cells inside the uterus during menstruation and early pregnancy.