Ataxia telangiectasia (AT) is a rare neurodegenerative, autosomal recessive disorder characterized by chromosome instability, radiosensitivity, immunodeficiency and a predisposition for cancer. Epidemiological studies have shown that AT heterozygotes have a predisposition for cancer, especially for breast cancer in women. The disease is caused by mutations in the ATM gene, leading to total loss of the ATM protein, which normally recognizes DNA damage, activates the DNA repair machinery and the cell cycle check points in order to minimize the risk of genetic damage. This review summarizes the clinical features of AT and the natural history of the disease and puts recent molecular advances into the context of the cellular and clinical phenotype.