The changes in population structure currently taking place in both developed and less-developed nations are part of a very long-term trend of demographic change that has yet to run its full course. The starting point of this trend is the complete rearrangement of demographic regimes characterised by significant declines in mortality coupled with widespread fertility control. This process started in a small group of European and non-European societies during the nineteenth century and by the second half of the twentieth century had spread to much of the world. It has brought with it significant economic and social implications for societies affected which have differed by the timing of the transition but not in their basic thrust. Eventually the demographic transition promises to bring with it very rapid and widespread ageing and, within a few decades, world population decline. Some of the long-term economic implications of this entire process are discussed in this article.