Epithelial-mesenchymal transition(EMT) is a process that epithelial cells lose polarity, become mesenchymal cells and acquire the ability of migration and invasion. It exists in many physiological and pathological processes. EMT is involved in a number of signal transduction pathways and performs different physiological functions. During the early stages of embryonic development, both EMT and MET(mesenchymal- epithelial transition) contribute to the formation and development of organs. Moreover, EMT can promote tumor metastasis. EMT also occurs in female mammalian reproduction. In the ovary, EMT is beneficial for repairing process following ovulation. During decidualization, MET may be required for successful uterine anchorage of the embryo. Placental development undergoes an EMT in order to facilitate nutrient and gas exchange between mother and fetus. The failure of EMT process may cause related reproductive diseases.