The ‘demographic transition’ refers to the fall of fertility and mortality from initially high to subsequent low levels and accompanying changes in the population. It began around 1800 with declining mortality in Europe, and is expected to be complete worldwide by 2100. In that time the global population will have risen tenfold, the ratio of elders to children will have risen by a factor of ten, longevity will have tripled, and fertility fallen from six births per woman to two. Individual and population ageing will pose many challenges, from life-cycle planning to the rising costs of health care and retirement.