Successful embryo implantation requires a receptive endometrium. Poor uterine receptivity can account for implantation failure in women who experience recurrent pregnancy loss or multiple rounds of unsuccessful in vitro fertilization cycles. Here, we demonstrate that the transcription factor Forkhead Box O1 (FOXO1) is a critical regulator of endometrial receptivity in vivo. Uterine ablation of Foxo1 using the progesterone receptor Cre (Pgr^Cre) mouse model resulted in infertility due to altered epithelial cell polarity and apoptosis, preventing the embryo from penetrating the luminal epithelium. Analysis of the uterine transcriptome after Foxo1 ablation identified alterations in gene expression for transcripts involved in the activation of cell invasion, molecular transport, apoptosis, β-catenin (CTNNB1) signaling pathway, and an increase in PGR signaling. The increase of PGR signaling was due to PGR expression being retained in the uterine epithelium during the window of receptivity. Constitutive expression of epithelial PGR during this receptive period inhibited expression of FOXO1 in the nucleus of the uterine epithelium. The reciprocal expression of PGR and FOXO1 was conserved in human endometrial samples during the proliferative and secretory phase. This demonstrates that expression of FOXO1 and the loss of PGR during the window of receptivity are interrelated and critical for embryo implantation. Human fertility is an inefficient process whereby 30% of pregnancies cease before the second trimester. The uterine environment plays a critical role in establishing and maintaining a successful pregnancy. Here, we show that the transcription factor Forkhead Box O1 (FOXO1) increases in the nuclei of endometrial epithelia of both human and mice during the time of embryo implantation, and thus, hypothesize that FOXO1 plays an important role in governing uterine receptivity. To test this hypothesis, we ablated Foxo1 in the mouse uterus. These mice were infertile due to defective implantation and invasion. The mutant mice demonstrated increased PGR signaling and retained PGR expression in the endometrial epithelia at the window of receptivity. By generating mice with deregulated uterine epithelial Pgr expression, we further demonstrated that constitutive expression of epithelial PGR during the receptive period inhibited the expression of FOXO1 in the uterine epithelium. Such reciprocal expression of PGR and FOXO1, similar to a Yin-and-Yang balance, was conserved in human endometrium during the proliferative and secretory phase, suggesting FOXO1 gain and PGR loss in the uterine epithelium is a marker and regulator of endometrial receptivity necessary for the establishment of pregnancy.