Aging of biological systems is a complex process that is controlled by both environmental factors and the genetic constitution of the individual. Although the molecular mechanisms have not been elucidated for any system in detail, it is clear that various genetic traits are involved in the modulation of life span. In particular, the genetic information located in the mitochondria has been identified as a major genetic component. Instabilities of the mitochondrial DNA (mtDNA) lead to mitochondrial dysfunction and increased oxidative stress. In some cases mtDNA instabilities are related to the activity of mobile genetic elements. In addition, nuclear genes appear to be crucially involved in mtDNA maintenance. Furthermore, the initial analysis of a few cloned nuclear genes affecting life span suggests a cellular machinery dealing with various stress situations as a major component involved in the genetic control of aging. This conclusion may hold true for all biological systems and be related to a unified mechanism of aging. However, in the various lineages this mechanism may be superimposed by other species or lineage-specific mechanisms.