It is essential for an embryo's further development that it generate a connection with the mother. The first stage of this process is implantation, a highly coordinated event that involves both embryonic and endometrial participation. A blastocyst may attach to the endometrium only during the "implantation window", when the uterus is receptive. A description of the molecular features of a receptive endometrium is the aim of this review. During the secretory phase, the endometrium synthesizes such cytokines as LIF (leukemia inhibitory factor), HB-EGF (heparin-binding epithelial growth factor), and TGF-alpha (transforming growth factor alpha) as well as special surface structures, such as integrins. There are two types of integrins: cycle dependent and constitutionally expressed. The coexpression of cycle-dependent integrins may mark the "implantation window". The endometrium of fertile and infertile women may vary in expression of the integrins. Insufficient synthesis of integrins may be caused by a luteal phase deficiency. E-cadherin and alpha-crystalin B play an important role during implantation. The expression of pinopodes on endometrial cells during the "implantation window" marks optimal uterine receptivity. The composition of all the factors mentioned above define the unique properties of the endometrium during the "implantation window".