Implantation of the blastocyst in the uterus is an evolutionary advance associated with viviparity and is required for sustained efficient nutrition and protection of the conceptus (embryo/fetus and associated extraembryonic membranes) during gestation. Because of its recent evolution, there is considerable diversity in types of implantation among eutherian mammals. Despite the diversity of implantation and placentation strategies, the initial events that occur between the trophoblast and maternal uterine endometrial lumenal epithelium (LE) are shared among species. Consequently, the comparative biology of implantation is useful to discover and understand the physiologic, hormonal, cellular and molecular mechanisms regulating implantation in mammals. By the blastocyst stage, the trophectoderm has acquired competence to attach and adhere to the endometrial LE, which appears to be the primary site of hormonally regulated uterine receptivity. If blastocyst development is synchronous with uterine receptivity, an adhesion cascade initiated at the apical surfaces of trophectoderm and LE results in definitive implantation. This review highlights new information on the mechanisms and factors regulating implantation that is focused primarily on the sheep.