Citizenship is the legal bond between an individual and a state; acquired by birth; naturalisation or other means according to national legislation.

Naturalisation is the process by which a state grants its citizenship through a formal act on the application of the individual concerned. Other ways of granting citizenship may include by marriage with a national; minors adopted by nationals and descendants of nationals born abroad returning to the country of origin of their ancestors.

Level of development is defined as the relative degree of development of a country as defined by statistical measures of three basis dimensions - health; education and standard of living.

Data on acquisition of citizenship until 2008 and data on loss of citizenship (from 2008 onwards) are collected on the basis of a gentleman's agreement. From 2008 on; acquisition of citizenship data by sex; age group and previous citizenship are collected under Regulation (EC) No 862/2007 of the European Parliament and of the Council on Community statistics on migration and international protection. Art 3.1(d) asks Member States to provide data on "persons having their usual residence in the territory of the Member State and having acquired during the reference year the citizenship of the Member State and having formerly held the citizenship of another Member State or a third country or having formerly been stateless; disaggregated by age and sex; and by the former citizenship of the persons concerned and by whether the person was formerly stateless".

Data on acquisitions of citizenship is available by age group; and sex; and by former citizenship and by level of human development of former citizenship. Data on loss of citizenship is available by sex and new citizenship.