In the past 50 years, the world accelerated its transition out of long-term demographic stability. As infant and child mortality rates fell, populations began to soar. In most countries, this growth led to falling fertility rates. Although fertility has fallen, the population continues to increase because of population momentum; it will eventually level off. In the meantime, demographic change has created a ‘bulge’ generation, which today appears in many countries as a large working-age population. This cohort will eventually become a large elderly population, in both developed and developing countries. Population growth has been the subject of great debate among economists and demographers. Until recently, most have agreed on a middle ground, in which population growth per se has no effect on economic growth. New evidence suggests that changes in the age structure of populations – in particular, a rising ratio of working-age to non-working-age individuals – leads to the possibility of more rapid economic growth, via both accounting and behavioural effects. The experiences of east Asia, Ireland and sub-Saharan Africa all serve as evidence of the effect of demographic change on economic growth (or lack thereof). Both internal migration (from rural to urban areas) and international migration complicate this picture. The overall implications of population growth for policy lie in the imperative for investments in health and education, and for sound policies related to labour, trade and retirement. Understanding future trends is essential for the development of good policy. Demographic projections can be quite reliable, but huge uncertainties – in the realms of health, changes in human life span, scientific advances, migration, global warming and wars – make overall predictions extremely uncertain.