Citizenship of the United States
A person who is considered a citizen by more than one nation has "dual citizenship". It is possible for a United States citizen to have dual citizenship; this can be achieved in various ways, such as by birth in the United States to a parent who is a citizen of a foreign country (or in certain circumstances the foreign nationality may be transmitted even by a grandparent) by birth in another country to a parent(s) who is/are a United States citizen/s, or by having parents who are citizens of different countries. Anyone who becomes a naturalized U.S. citizen is required to renounce any prior "allegiance" to other countries during the naturalization ceremony; however, this renunciation of allegiance renunciation of citizenship to those countries.
European Free Trade Association
Since each EFTA and EU country can make its own citizenship laws, dual citizenship is not always possible. Of the EFTA countries, Iceland and Switzerland allow it (in Switzerland, the conditions for the naturalization of immigrants vary regionally), but Norway only in exceptional cases, and Liechtenstein only for citizens by descent, but not for foreigners wanting to naturalize.