This chapter provides data to identify which day(s) of development the human embryo should be replaced in the uterus. By transferring the embryo post-compaction at the blastocyst stage, one not only synchronizes embryo development with the female reproductive tract, but also places the embryo in the uterus at a time when there are greatly reduced uterine contractions, thereby negating the possibility of the embryo being expelled. The first prospective randomized trial comparing human embryo transfer on either day 3 or day 5 in good prognosis patients revealed a significantly higher implantation and pregnancy rate when transfer occurred at the blastocyst stage. With the recent improvements in the in vitro fertilization (IVF) laboratory, combined with patient stimulation regimens designed to generate fewer, healthier oocytes, single embryo transfer (SET) is now a feasible patient treatment. The move to SET is especially relevant to good prognosis patients and oocyte donation programs.