The paper challenges the view that the late twentieth century is the ‘age of migration’. For developing countries, flows of out-migrants are small compared with population growth, although in developed countries the stock of immigrants increased in proportion to the total population between 1965 and 1990. Despite the importance of refugee movement, the main force for international migration is economic. Why do not more people migrate (internally and internationally) to take advantage of potential economic gains? For international migration, one deterrent is institutional barriers against uncontrolled immigration. Different interest groups stand to gain or lose from increased migration. The income-enhancing effects of unhindered international labour migration, measured jointly for sending and receiving countries and by extension globally, should be very large. Even partial liberalization of immigration to industrialized countries would serve developing countries well. In industrialized countries, however, there is concern about the effect of massive labour inflows on the ethnic, religious and cultural composition of the population and its social cohesion. In some countries, migration is leading to greater ethnic mingling; in others there is a recrudescence of nationalistic aspirations for independent statehood with ethnically homogeneous populations, or to preserve the advantages of economically successful subregions.