Social analysts have generally paid too little attention to demographic trends, as conventional wisdom holds that rapid population growth inhibits improvement in living standards. This short monograph by three authors associated with Harvard's School of Public Health attempts to clarify the complexities of demographic change and economic growth. Modern societies have typically passed through a demographic transition in which the labor force grows more rapidly than total population because a decline in mortality precedes a decline in fertility. In the right policy environment -- one conducive to education and to saving -- this transition creates the potential for exceptionally rapid economic growth, as has been experienced in recent decades by Ireland and by several East Asian countries. The authors draw attention to this opportunity for many developing countries over the next 20 years, urging these countries to take advantage of the situation. Similarly, rich countries need to prepare for significant aging of their populations and, in some cases, for shrinking labor forces -- an altogether new experience in the modern era.