Ageing, because of its complexity, has long been regarded as intractable to experimental analysis and medical intervention. However, this pessimistic view has been overturned by the recent discovery that mutations in single genes can increase healthy lifespan in laboratory animals. Furthermore, the mechanisms at work are conserved over large evolutionary distances, with population--genetic association studies implicating similar processes in humans. Nutrient and stress--sensing pathways have proved to play a key role, through mechanisms that are the subject of current investigation. Modulation of the activity of these pathways can also ameliorate pathology in several animal models of human age--related diseases. These findings are pointing to the prospect of a broad--spectrum, preventative medicine for the diseases of ageing. Academy of Medical Sciences Sackler Lecture 2012, given by Professor Linda Partridge on the science of ageing at the University of Cambridge. Watch an interview with the speaker at http://ow.ly/eMGwl. To find out about the Sacker Lecture 2013, visit our website at http://ow.ly/eMGBE.