Ataxia telangiectasia (AT) is an incurable, rare autosomal recessive genetic disorder which affects approximately one in 40,000-100,000 and the carrier frequency is estimated to be 1:100-200. In the United Kingdom it affects 1 in 300,000 live births amounting to about 5 or 6 cases per year. Although the medial survival rate of affected persons is between 19-25 years, due to genetic heterogeneity, the survival rate can be fairly variable. In rare cases, however, patients survive into their third decade. The longest living patient on record died at age 34 years. Although the disease may have been identified earlier, according to PubMed the first published report on AT was in 1958. These authors described the disease to be of a slowly progressive type; the patient suffered from sinopulmonary infection, which must have been due to a deficiency in the immune system, that was discovered later in AT patients. Since this publication, 4720 research papers including a number of excellent reviews have appeared (PubMed data, September, 2008). Results from these studies have provided exciting information on a multiplicity of hitherto unknown complex processes of sensing DNA damage, cell signalling, protein/protein interactions and their post-translational processing, involving activities from control of redox state to repair of double strand DNA breaks (DSB), with outcomes that include apoptosis and carcinogenesis. Despite a huge volume of research already carried out, we are still some distance away from a complete understanding of the molecular mechanisms of AT, and there is no evidence that interest in this disease is waning. Hence this book should provide both expert and novice researchers in the field with an excellent overview of the current status of research and pointers to future research goals.