Each country has different requirements for citizenship, as well as different policies regarding dual citizenship. An Australian study estimated that 4-5 million Australians (up to 25% of the Australian population - by far the largest group at 1.6 million of these, from the UK) had dual citizenship in 2000. An estimated sixty percent of Swiss nationals living abroad in 1998 were dual citizens.
Approximately 89 countries in the world officially allow some form of dual or multiple citizenship.In the United States it is estimated that millions of Americans are also citizens of other countries. Although Germany has a very restrictive nationality law, it does allow dual citizenship under certain circumstances and the number of dual-citizens was estimated at 1.2 million in 1994. See German nationality law
The Republic of Ireland extends its citizenship laws to Northern Ireland, which is part of the United Kingdom. Therefore, most people born in Northern Ireland who are British citizens just like people born else where in the United Kingdom may, if they wish, exercise an entitlement to Irish citizenship by simply applying for an Irish passport. Most people born in Northern Ireland may hold either a British passport or an Irish passport or both British passport and Irish Passports. See Irish nationality law and British nationality law.
 Sub-national citizenship
* The U.S. Constitution also recognizes "dual citizenship" with each of the U.S. states. State citizenship, however, is informal in practice, and is obtained simply by taking up residence in any given state. (Residents of the District of Columbia do not have dual citizenship, as it is not a state.) There is now almost no legal bearing brought by such an arrangement, except possibly for interstate extradition.
* Switzerland has a three tier system of citizenship - Confederation, canton and commune (municipality).
* Although considered part of the United Kingdom for British nationality purposes, the Crown Dependencies of Jersey, Guernsey and the Isle of Man have local legislation restricting certain employment and housing rights to those with "local status".
* British Overseas Territories do not have individual 'citizenships' but they do have under their immigration laws a concept of Belonger status which is equivalent. Also see British Overseas Territories citizen
* The Australian territory of Norfolk Island has immigration laws which restrict residence in the territory to those with "local status". Most Norfolk Islanders are Australian citizens
* The Hong Kong and Macao Special Administrative Regions of the Peoples Republic of China make a distinction in their immigration laws between Chinese citizens with the right to reside in the territory and those without. Chinese citizens with right of abode in these territories can hold a different type of Chinese passport which gives more favourable travel rights internationally.
* People from Åland have joint regional (Åland) and national (Finnish) citizenship. People with Ålandic citizenship (hembygdsrätt) have the right to buy property and setup a business on Åland, which Finns without regional citizenship cannot. Finns can get Ålandic citizenship after living on the islands after five years and Ålanders loses their regional citizenship after living on the Finnish mainland after five years. See  and 
* Prior to the dissolution of Czechoslovakia in 1993, Czechoslovak citizens also possessed an internal citizenship of either the Czech or Slovak Republic.
* Before the break-up of Yugoslavia in 1991, Yugoslav citizens possessed an internal citizenship of their own republic (e.g. Serbia, Croatia, Slovenia etc.) as well as Yugoslav citizenship.