Our knowledge relating implantation and the endometrium is largely descriptive at present. In the absence of a unifying hypothesis, I describe endometrial receptivity for implantation. Tissue component data are described, first relating to whole tissue, then isolated glands and stroma, vessels, the luminal epithelium, and infiltrating leukocytes. Finally, I consider trophoblast-endometrial interaction. Stimulatory and inhibitory factors impinging on a proteolytic cascade must be related to the process of invasion of the trophoblast. Only early data are available, as the description of this fascinating process unfolds.