Ageing is not a random process. Biological ageing processes are instead regulated by metabolic and genetic mechanisms. Single gene mutations can markedly extend the life span of various organisms. The biology of ageing can be investigated in simple yeast cells, flies, round worms, and also in mice. Gene mutations that extend life span also protect against age-associated diseases such as neurodegeneration, cancer, heart disease, and diabetes. A deeper understanding of molecular mechanisms of longevity can open new avenues for therapies or prevention of these highly relevant diseases.