Human reproduction, as all biological systems, is characterised by a large level of variability. In this field, the variability is observed as a large difference in implantation potential of human embryos developing in vitro, despite similarities in observable parameters such as rate of 5 development and morphology of these embryos. One of the underlying factors that determines developmental potential in these embryos is the availability of energy in the form of adenosine trisphosphate (ATP) for development. Here, we suggest that, despite the evidence suggesting that mitochondrial metabolism is relatively inactive during preimplantation embryo development, aerobic (mitochondrial) metabolism contributes a major role in the supply of ATP. A second 10 pathway, anaerobic respiration, is also active and the two pathways work in synchrony to supply all the ATP necessary. We discuss the differences in the two forms of energy production and suggest that, although anaerobic respiration can supplement deficiencies in the energy supply in the shortterm, this is not sufficient to substitute for aerobic respiration over long periods. Therefore, we suggest that deficiencies in levels of aerobic respiration can explain variability in the implantation 15 potential of apparently equivalent embryos.