Implantation is essential for successful reproduction. In humans, uterine receptivity is restricted to the mid-secretory phase, days 19 to 24 of the menstrual cycle; and this period is called the window of implantation (WOI). Large populations of decidual leukocytes infiltrate the implantation site, and the levels of cytokines, prostaglandins (PGs), and leukocytes are up-regulated in the endometrium during implantation. Implantation is like a local wound healing process which is characterized by a strong Th1, pro-inflammatory response in which high levels of pro-inflammatory cytokines such as IL-6, LIF, IL-8, and TNFα are involved. In addition, cyclooxygenase (COX)-2 expression increases between days 2 to 5, suggesting that PGs are necessary for the process of stromal cell decidualization in early pregnancy. Recently, it was reported that endometrial biopsies taken during spontaneous cycles that preceding IVF treatment improved the rate of implantation, clinical pregnancies, and live births in patients with recurrent implantation failure. These results suggest an ininflammatory environment is not adequately induced in some IVF patients with recurrent implantation failure in the absence of local injury provoked by biopsy treatment, further suggesting that uterine receptivity is mediated by the expression of molecules associated with a pro-inflammatory process.