Mitochondrial DNA (mtDNA) forms a semi-autonomous asexually reproducing genome in eukaryotic organisms. It plays an essential role in the life cycle through the control of energy production, by the inherently dangerous process of oxidative phosphorylation. The asymmetric nature of its inheritance--almost exclusively through the female--imposes different evolutionary constraints on males and females, and may lie at the heart of anisogamy. This review examines the implications of recent findings on the biology of mtDNA for reproduction and inheritance in mammals.