This paper reviews immigration trends and their economic impacts in a number of OECD countries. While migration systems present similarities across countries, institutional arrangements vary widely and impact on the size and composition of migration flows. Some of the main factors driving immigration are then briefly discussed. The paper also considers the economic, fiscal and social implications of immigration. The study suggests that immigration can confer small net gains to the host country. However, the benefits are not necessarily evenly distributed and some groups, in particular those whose labour is substitutable with immigrants may lose, calling for a smooth working of labour and product markets in OECD countries. The paper also claims that, while migration can partly offset slower growing or declining OECD populations, it cannot provide by itself a solution to the budgetary implications of ageing populations. Finally, the paper touches on some development issues, such as ...