Implantation window is the transient period when the embryos develop into blastocysts and the uterus differentiates into the receptive state synchronically. Estrogen and progesterone are the comprehensive regulating molecules during this process. They influence the proliferation and differentiation of multiple cell types in the uterus through the modulation of various local-signaling molecules. Uterus and blastocyst interact by the paracrine effects of prostaglandin, histamine, calcitonin, cytokines and growth factors at implantation window. This molecular cross-talk modulates the interaction between trophectoderm and uterine luminal epithelium. Once the implantation window is open, it then switches into unreceptive state spontaneously.