A comprehensive transcriptional map of human preimplantation development has been revealed through single-cell RNA sequencing, analyzing 1,529 individual cells from 88 human preimplantation embryos. The study shows that cells undergo an intermediate state of co-expression of lineage-specific genes, followed by the concurrent establishment of the trophectoderm, epiblast, and primitive endoderm lineages, coinciding with blastocyst formation. Female cells in all three lineages achieve X chromosome dosage compensation prior to implantation. Unlike in mice, XIST is transcribed from both X chromosomes throughout the expression dampening process, and X chromosome genes maintain biallelic expression while dosage compensation proceeds. The study highlights the importance of this transcriptional atlas for future research on human development and stem cell research. The findings reveal that X chromosome dosage compensation occurs in all three lineages, with a gradual decrease in female X chromosome overexpression from E4 to E7. XIST and XACT expression were also analyzed, showing upregulation of XIST in female cells and activation of XACT at E4 in both sexes. Biallelic expression of dose-compensated genes was observed, indicating that X chromosome dosage compensation in the human preimplantation embryo is achieved by reducing the expression of both X chromosomes, rather than silencing one. The study also identified novel and less-studied genes that may be important for preimplantation development. The findings provide a detailed understanding of the lineage specification and X chromosome dynamics in human preimplantation embryos.A comprehensive transcriptional map of human preimplantation development has been revealed through single-cell RNA sequencing, analyzing 1,529 individual cells from 88 human preimplantation embryos. The study shows that cells undergo an intermediate state of co-expression of lineage-specific genes, followed by the concurrent establishment of the trophectoderm, epiblast, and primitive endoderm lineages, coinciding with blastocyst formation. Female cells in all three lineages achieve X chromosome dosage compensation prior to implantation. Unlike in mice, XIST is transcribed from both X chromosomes throughout the expression dampening process, and X chromosome genes maintain biallelic expression while dosage compensation proceeds. The study highlights the importance of this transcriptional atlas for future research on human development and stem cell research. The findings reveal that X chromosome dosage compensation occurs in all three lineages, with a gradual decrease in female X chromosome overexpression from E4 to E7. XIST and XACT expression were also analyzed, showing upregulation of XIST in female cells and activation of XACT at E4 in both sexes. Biallelic expression of dose-compensated genes was observed, indicating that X chromosome dosage compensation in the human preimplantation embryo is achieved by reducing the expression of both X chromosomes, rather than silencing one. The study also identified novel and less-studied genes that may be important for preimplantation development. The findings provide a detailed understanding of the lineage specification and X chromosome dynamics in human preimplantation embryos.