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 considered to be a tumour suppressor, ATM signaling may be advantageous to cancer cells in some settings, particularly in resistance to radio- and chemotherapeutic treatment. For this reason, the use of ATM inhibitors in cancer therapy is under exploration.