Mitochondria are not only the main source of energy for most eukaryotic cells, but also the main source of free radicals. These reactive molecules can damage all components of a cell such as membranes, proteins and DNA. Therefore they have long been suspected to be involved in the biological aging process. The fact that mitochondria posses their own genetic material (mtDNA) and that they only have a limited arsenal of DNA repair processes makes them one of the prime targets for reactive oxygen species. The idea that genetically damaged mitochondria accumulate with time and are causally responsible for the aging phenotype via a disturbed energy budget is at the core of the so called mitochondrial theory of aging. In recent years this idea has gained impetus from the discovery of mitochondrial diseases and mtDNA deletions in old organisms. However, there are still many open questions regarding the mechanism of the accumulation of these deletions and their physiological relevance. This review is therefore intended to give an overview of the current state of the mitochondrial theory of aging and to discuss some recent experimental findings.