Endometrial receptivity to human embryo implantation can be simply defined as the ability for the maternal endometrium to support appropriate attachment, invasion, and maintenance of the early human embryo. In contrast to the simplicity of definition, understanding the mechanisms involved in early human embryo implantation are severely limited due to differences between humans and experimental animals, as well as ethical, moral, and technological barriers to direct observation of human implantation in vivo and ex vivo. There is no doubt, however, that the human endometrium has a limited temporal window of receptivity during which normal embryo implantation can occur, and that this time window is largely driven by the action of progesterone. Given the critical nature of endometrial receptivity and implantation in fertility and pregnancy health, it remains an important area of study despite the barriers. This chapter reviews our understanding of endometrial receptivity as gleaned from animal studies, observational studies in women attempting conception with or without medical assistance, and limited experimental studies in women undergoing assisted reproduction. We conclude the chapter with a brief description and appraisal of proposed clinical testing for altered receptivity.