Controlled immune cell access to the pregnant uterus may be one of the mechanisms involved in maternal tolerance leading to the presence of a selected population of immune cells at the maternal/fetal interface. The molecular determinants responsible for coordinating recruitment of leukocytes include the cellular adhesion molecules and members of the chemokine superfamily. During the critical period of initial placenta development in the mouse an elegantly orchestrated progression of leukocyte homing events in the decidua basalis has been described. Moreover, the maternal/fetal interface displays an unparalleled compartmentalization of microdomains associated with highly differentiated vessels expressing vascular addressins in nonoverlapping patterns. These expression patterns are functionally correlated with the distinct localization of uterine NK cells, monocyte-like cells and neutrophils. Switches in vascular specificity and the partial loss of microenvironmental specialization during the second half of mouse development have been shown to parallel dramatic changes in the populations of leukocytes recruited to the maternal/fetal interface. Recently, complex expression patterns of chemokines and their receptors were described in the human pregnant uterus suggesting that along with adhesion molecules these determinants are critical for leukocyte trafficking to the pregnant uterus.