UN Department of Economic and Social Affairs – Population Division
United Nations projections indicate that over the next 50 years, the populations of virtually all countries of Europe as well as Japan will face population decline and population ageing. The new challenges of declining and ageing populations will require comprehensive reassessments of many established policies and programmes, including those relating to international migration.
Focusing on these two striking and critical population trends, the report considers replacement migration for eight low-fertility countries (France, Germany, Italy, Japan, Republic of Korea, Russian Federation, United Kingdom and United States) and two regions (Europe and the European Union). Replacement migration refers to the international migration that a country would need to offset population decline and population ageing resulting from low fertility and mortality rates.
http://www.un.org/esa/population/publications/migration/migration.htm