Quota, nationality or other restrictions on immigrants may be imposed by national governments. Restrictions may be based on employment opportunities and also sometimes on housing availability. Immigrants may effectively or explicitly be discriminated against on the grounds of colour or race or nationality if the country has preferential agreements with certain other countries.
The four traditional immigration countries, the USA, Canada, Australia and New Zealand, all have systems of restrictions. However, Israel, the Republic of South Africa, the USSR, and numerous Latin American countries also have highly selective immigration policies. The number of persons who would like to immigrate and whose applications are pending for long periods, in addition to those who are barred by restrictions, possibly numbers in the millions world-wide.