IN the next half century the size and distribution of the world's population will change rapidly. These changes will bring new demographic problems, and shift both the locus and form of old ones. Areas of Europe and Europe overseas in which technological civilization is most fully developed face slowing growth and perhaps gradual population decline. The phase of rapid growth which formerly characterized their populations is shifting to less fully developed areas such as Eastern Europe, the Soviet Union, and Japan. There, declines in fertility are well established but have not yet overtaken those in mortality which modernization has induced; hence growth is rapid. In the remainder of the world, actual population change ranges from apparent decline in parts of Central Africa to very rapid increases in many areas of the Near and Far East and of Latin America. However, virtually all of these populations have high birth and death rates. In any of them the application of established techniques for the reduction of mortality would bring about a very rapid population growth. Irrespective of their past actual growth, such populations have the potentiality for rapid future growth. Populations with high growth potentials include most of those of Latin America, and, except for Japan, virtually all of those in which non-European cultures are dominant-in short, virtually all of the populations in the technologically undeveloped regions of the world. Many of the world's undeveloped regions could absorb substantial growth readily enough. Throughout large parts of South America, Africa, and the Middle East, developments that would foster rapid population increase would also elicit the economic 1 From the Office of Population Research, School of Public and International Affairs, Princeton University.