The study of gene expression in human preimplantation embryos is establishing itself as a necessary dimension of developmental biology and medical genetics. Transcripts identified in human preimplantation embryos include housekeeping genes, transcription and growth factor genes, sex-determining genes, tissue-specific genes and novel genes, as well as genes of unknown function. Strategies are being developed which will eventually permit the most sophisticated gene expression studies on single human embryos of co-ordinated transcription and translational regulation. There is both a need for international co-operation for the systematic construction of expression maps and a need to establish databases of expression patterns during different stages of human development. Understanding how genes are regulated in humans is essential for understanding both normal development and disease. Until recently, studies of gene expression and regulation during embryogenesis were almost exclusively limited to prokaryotes and to eukaryotes other than man. The introduction of artificial reproductive technologies in conjunction with the development of recombinant molecular technologies applicable to single cells has made possible the study of human development at its earliest stages (Pergament and Bonnicksen, 1994). Although there are still enormous technical challenges, robust strategies have been, and continue to be, developed for connecting DNA sequence to such endophenotypes as timing and level of genes expression at the single cell level. Questions currently being asked in human developmental genetic studies concern the pronucleus, the zygote and the preimplantation embryo: what genes are expressed? When are they expressed? What functions do they perform and how, in sequence or in combination? And, what elements control and regulate their expression? This review provides an overview of current knowledge about the expression of different embryonic genes during early human development and discusses future prospects, which includes a need for international co-operation similar to the Human Genome Project.