Mutations in mitochondrial DNA (mtDNA) are undoubtedly associated with a diverse spectrum of human disorders. More controversially, it has been claimed that they accumulate during ageing, and that they are responsible for an age-related decline in bioenergetic function and tissue viability. Here, we review the evidence for this assertion, concluding that claims for the age-accumulation of mtDNA mutations are based largely on non-quantitative methods, and that no clear, functional deficit of mitochondrial respiration has been shown to result from such lesions in aged individuals. The mitochondrial theory of ageing, however attractive in principle, is supported by very little hard evidence.