This chapter summarizes the hormonal regulation and molecular bases of endometrial receptivity, its clinical implication and the possible models to study this complex process and to develop functional assays in vitro. Cyclic changes of the endometrium have been well described at the light microscopy level. A large amount of research worldwide has been focused on the problem of finding a specific marker(s) of uterine (endometrial) receptivity-biomarkers that can be useful for the diagnosis and treatment of couples with infertility of endometrial origin. The main techniques used to study the features of the receptive endometrium include microscopy for endometrial morphology, quantitative PCR, in situ hybridization and gene expression microarrays in endometrial biopsy. The ultimate in vitro model to study endometrial receptivity and embryonic implantation would contain all the cell types of the endometrium so that the complex interactions between the maternal tissue and the blastocyst could be characterized.