Publisher Summary This chapter summarizes the evidence for or against the notions that DNA damage accumulates during cellular aging, which is because of decreased repair capacity in old age. The species specific longevity or maximum achievable life span is a function of a species' capacity to repair this damage and to maintain efficiently its genomic integrity. The removal or repair of damage to DNA is a highly complex process, involving a series of interconnected and interacting steps and has strong intragenomic heterogeneity between active and inactive regions of the genome. The development of methods for measuring one or more kinds of DNA damage and repair even in single-copy genes, which can be further amplified by polymerase chain reaction (PCR), enables DNA damage and repair at more specific levels instead of measuring gross changes in the genome during cellular aging. A combination of biochemical, molecular, and immunochemical methods for the detection of specific DNA lesions in the genome helps to establish the types of DNA damage relevant to the phenomenon of aging.