Ultra-high dose rate (FLASH) radiotherapy is a new way of treating tumours caused by cancer. Higher doses of radiotherapy are associated with trauma to the healthy tissue surrounding the tumour, whereas FLASH radiotherapy demonstrates a sparing effect of the healthy tissues without compromising the anti-tumour action. Dr Kristoffer Petersson at the Oxford Institute for Radiation Oncology, University of Oxford, along with collaborators Joseph D. Wilson, Ester M. Hammond and Geoff S. Higgins, review the available data on FLASH radiotherapy and its clinical potential in the treatment of cancer.