In both mice and humans, 15-50% of embryos die during the preimplantation period from mechanisms that are largely unknown. Two major criteria predict preimplantation embryo quality, the rate of development and the degree of fragmentation. We review evidence that both of these criteria have a genetic basis. Rate of development and subsequent embryo survival are controlled by a gene, Ped, we discovered in the mouse. Although progress is being made in the search for the human homologue of the mouse Ped gene, it has not yet been identified. Fragmentation, observed in both mouse and human embryos, is probably the result of apoptosis. We analysed transcription of two genes that regulate apoptosis, bcl-2 and bax, and found that both are transcribed in mouse and human preimplantation embryos. Overall, the literature reviewed and new data presented in this paper support the concept that there is a genetic basis for preimplantation egg and embryo survival.