Migration Resources.

Migration Policy Institute (MPI)
Independent, nonpartisan, nonprofit think tank in Washington, DC dedicated to analysis of the movement of people worldwide.

International Organization for Migration
A leading inter-governmental organization in the field of migration.

Europe
Eurostat
The statistical office of the European Union which provides statistics at European level that enable comparisons between countries and regions.

United Kingdom
The Office for National Statistics 
the UK’s largest independent producer of official statistics and is the recognized national statistical institute for the UK. Ues the "Browse by theme" section to see the subsection on Migration under Populations.

Germany
Federal Office of Migration and Refugees
Information on residency and immigration of the German citizenry.

Italy
The Italian National Institute of Statistics (ISTAT.IT) 
A public research organization that has been the main producer of official statistics in the service of citizens and policy-makers in Italy since 1926.