Evolutionary theory and empirical evidence from many lines of research suggest that ageing is a process of gradual accumulation of damage in cells and tissues of the body, leading eventually to frailty and increased risk from a spectrum of age-associated diseases. There are multiple kinds of damage that affect cells, ranging from mutations in DNA to oxidative attack on proteins by chemical by-products of normal cellular metabolism. In some ways the surprising thing is not that we age, but that we live as long as we do. The key to understanding longevity lies in the network of cell maintenance systems that cooperate to slow the accumulation of damage. Research has shown that long-lived species carry out cellular maintenance better than short-lived species, suggesting that enhancement of the body's natural maintenance systems may postpone aspects of ageing. Recognition that ageing results from accumulation of damage also points to a role for lifestyle interventions (e.g. nutrition and exercise) to help prevent damage or promote repair.