This is a guide to the science driving biology’s biggest story: why we get old, and how we can stop it. Ageing is so deeply ingrained in human experience that we never think to ask: is it necessary? Biologists, on the other hand, have been investigating that question for years. This work introduces us to the cutting-edge research that is paving the way for a revolution in medicine. It takes us inside the laboratories where scientists are studying every aspect of the cell – DNA, mitochondria, stems cells, our immune systems, even age genes that have helped animals to a tenfold increase in lifespan – all in an effort to forestall or reverse the body’s decline. Computational biologist Andrew Steele offers reality-based hope, explaining what is happening as we age and practical ways we can help slow down the process.