Place of birth and nationality are the two criteria commonly used by OECD countries to define their immigrant population. According to the first criterion, migrants are persons residing in a country but born in another. According to the second criterion, migrants are residents who have a foreign nationality and may include persons born in the host country. Cross-country differences between the size of the foreign-born population and that of the foreign population depend on the rules governing the acquisition of citizenship in each country. In general, the foreign-born population is substantially larger than the foreign population. More information on the origin and characteristics (age, gender, educational level, duration of stay and labour market outcomes) of the immigrant population in OECD countries can be found in the publication A Profile of Immigrant Populations in the 21th Century.