ATM alterations have been reported as germline variants which predispose to inherited cancer syndromes and as somatic (acquired) variants in tumors. ATM is part of many signalling networks, including cell metabolism and growth, oxidative stress, and chromatin remodelling, all of which can affect cancer progression. Although ATM is rightly considered to be a tumour suppressor, ATM signalling can also be advantageous to cancer cells, particularly in resistance to radio- and chemotherapeutic treatment. For this reason, ATM inhibitors have been developed for use in cancer therapy.