No other organ in the body undergoes such an invasion of selective cells (leukocytes) and release of homing molecules, CAMs, proinflammatory cytokines, and mediators or undergoes similar extensive remodeling of tissues over such a short period of time as the pregnant uterus. This is especially interesting, as an infectious process involving microorganisms does not exist in a healthy pregnancy and delivery. Furthermore, after delivery of the baby and placenta, the uterus involutes and returns to its normal monthly cycling, and most of the leukocytes are swept away or leave. In this review, we discuss leukocyte infiltration and recruitment and the potential roles of each subpopulation of leukocytes in relation to pregnancy and the problems of preterm birth, pre-eclampsia, and intrauterine growth restriction.