Place of birth and nationality are the two criteria most commonly used by OECD countries to define their immigrant population. Based on the first criterion, migrants are persons residing in a country but born in another, i.e. first-generation migrants. According to the second criterion, migrants are residents who have the nationality of their home country, 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, estimates of the foreign-born population are substantially higher than those based on nationality. While different national definitions have traditionally limited cross-country comparability of the stock of migrants in different OECD countries, this issue of Society at a Glance presents for the first time comparable data of the foreign-born population derived from population censuses (Dumont and Lemaître, 2005).