Diagnostic medical x-ray examinations, including CT scans, play a vital role in modern healthcare. X-rays, as a form of ionising radiation, are associated with an increased risk of developing cancer. Information on the cancer risks from low doses of ionising radiation, including diagnostic x-rays, is limited, however. At Newcastle University, we have assembled cohorts of people who have undergone medical x-ray procedures (CT scans and cardiac catheterizations) while aged under 22 years. These cohorts will be pooled with cohorts from other European countries to increase statistical power, providing unprecedented ability to determine the excess cancer risks at low doses. This research will aid clinicians, radiographers and physicists in the effective use of diagnostic x-ray imaging, including the processes of justification (weighing up the advantages of an x-ray exposure with the risks) and optimisation (reducing doses to as a low as reasonably achievable).