The latest Health Report of the World Health Organization predicts a significant increase in the age of human populations over the next two decades. In the developed world, at least 20% of the population will be older than 65 years. This development together with the as yet unknown etiology of many neurodegenerative disorders has caused an increased interest in the biology and pathophysiology of mitochondria. Dysfunction of mitochondria has been linked to both normal aging and neurodegenerative disorders, with the latter occurring much more frequently at higher age. Specifically, genetic defects in mitochondria have been shown to accumulate during life, and certain mutations of mitochondrial genes have been implicated in the etiology of Parkinson's and Alzheimer's diseases. In addition, a large number of new mitochondrial diseases have been identified following the first description of mitochondrial mutations 10 years ago. While there can be little doubt that DNA defects of mitochondria play a role in aging, specific mutations of mitochondrial genes underlying Parkinson's or Alzheimer's diseases remain to be identified. There is evidence, however, that mutations of the mitochondrial genome may increase the susceptibility to neurodegeneration.