It appears that not only mitochondria, but also mtDNA, are important to early developmental outcome. It is unlikely that oocyte mitochondria provide significant amounts of energy but, as the carriers of mtDNA, they ensure that sufficient mtDNA copy is present. The presence of sufficient mtDNA copy number likely promotes mitochondrial function by generating efficient electron transport chains that maintain efficient mitochondrial membrane potentials and this ensures that the mitochondria can mediate important functions that are not limited to energy production. Indeed, the nucleus and the mitochondrial genome interact at key stages during development. When this relationship breaks down, cell function is dissipated. Supplementation of mitochondrial deficient oocytes suggestions that this is a mtDNA mediated event as mtDNA is replicated but the number of injected mitochondria does not increase (Figure 14). Consequently, mitochondrial function and mtDNA are essential investments in oocyte quality, fertilisation outcome, embryo development and offspring survival and well being. It is, therefore, imperative that they are given due consideration in quality assessments in assisted reproductive laboratories.