We are constantly exposed to sources of agents that directly damage the genetic material. This exposure comes from environmental sources but also from within our own organisms. DNA damage occurs at a high frequency due to metabolic processes and environmental factors such as various exposures and the intake of food and drugs. The stability and correct function of the DNA is necessary for normal cellular functions and there is good evidence that damage to the DNA can lead to cellular dysfunction, cancer and other diseases, or cell death. To avoid or minimize the damage to DNA we have evolved an elaborate set of DNA repair pathways that survey the DNA and fix the errors. There are several human diseases that are known to be defective in these repair pathways, and the accumulation of DNA damage with time in their genome may then be the cause of the associated high incidence of cancer or of an expedited ageing process. The prevention and/or repair of DNA damage thus represent major concerns in biology and medicine.