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Multiple citizenship is a status in which a person is concurrently regarded as a citizen under the laws of more than one state. Multiple citizenships exist because different countries use different, and not necessarily mutually exclusive, citizenship requirements. Colloquial speech refers to people "holding" multiple citizenship but technically each nation is making a claim that this person be considered its national. For this reason, it is possible that a person is a citizen of one, none, or multiple countries. It is possible to obtain a second citizenship through a financial investment in a few countries, e.g., Austria, Cyprus, Dominica and St. Kitts & Nevis.

Individual countries follow their own rationales in establishing their criteria for citizenship. Common reasons to bestow citizenship are: At least one parent is a citizen (jus sanguinis), birth takes place on the country's territory (jus soli), or marriage to a person holding the citizenship (jure matrimonii). In many countries, citizenship can be attained through naturalization. Once citizenship is bestowed, the bestowing country may or may not consider a voluntary renunciation of citizenship to be valid.

Some countries consider multiple citizenship undesirable and take measures to prevent it. This may take the following forms:

However, some countries do recognise foreign nationalities of their citizens. It is possible to become a citizen of multiple countries even if some or all of these countries forbid dual or multiple citizenship. For example, Germany and Austria usually do not allow dual citizenship except for persons who obtain more than one citizenship at the time of birth, but Germans and Austrians can apply for a permit to keep their citizenship ("Beibehaltungsgenehmigung") before taking a second one (e.g., Arnold Schwarzenegger holds Austrian and US citizenship). Spain allows dual citizenship with some Ibero-American countries (Argentina, Bolivia, Chile, Costa Rica, the Dominican Republic, Ecuador, Guatemala, Honduras, Nicaragua, Paraguay, and Peru), Andorra, Portugal, the Philippines and Equatorial Guinea. Spain does not require citizens of these countries who wish to naturalise to renounce their existing citizenship.

Some countries consider multiple citizenship desirable as it increases opportunities for their citizens to compete globally, and/or have taken active steps towards permitting multiple citizenship in recent years (e.g., Switzerland since 1 January 1992 and Australia since 4 April 2002). India has introduced a form of overseas citizenship, which stops just short of full dual citizenship and is in all aspects, like Permanent Residency. Many countries, even those that permit multiple citizenship, do not recognise multiple citizenship under their laws: individuals are treated either as citizens of that country or not, and their citizenship with respect to other countries is considered to have no bearing. This can mean (e.g., in Iran, Mexico, many Arab countries, and former Soviet republics) that consular officials abroad may not have access to their citizens if they also hold local citizenship. Some countries provide access for consular officials as a matter of courtesy, but do not accept any obligation to do so under international consular agreements. The right of countries to act in this fashion is protected via the Master Nationality Rule. In popular discourse, reference to countries that recognise multiple citizenship may refer only to the lack of any specific statute forbidding multiple citizenship (leaving aside the difficulties of enforcing such statutes).

Each country has different requirements for citizenship, as well as different policies regarding dual citizenship. These laws sometimes leave gaps where the acquisition of other citizenships does not render the original citizenship invalid, creating a possible situation for an individual to hold two or more nationalities.

The Republic of Ireland frames its citizenship laws as relating to "the island of Ireland", thereby extending them to Northern Ireland, which is part of the United Kingdom. Therefore, anyone born in Northern Ireland who meets the requirements for being an Irish citizen through birth on the island of Ireland (or a child born outside of Ireland but with a qualifying parent) may exercise an entitlement to Irish citizenship by acting in such a way that only an Irish citizen is entitled to do (such as applying for an Irish passport). Conversely, that such a person has not acted in this way does not necessarily mean that they are not an Irish citizen. See Irish nationality law and British nationality law. People born in Northern Ireland are British citizens on the same basis as people born elsewhere in the United Kingdom. People born in Northern Ireland can hold either a British Passport or an Irish Passport, or both if they so choose.

In the case of naturalisation, some countries require applicants of naturalisation to renounce their former citizenship. This renunciation may not be recognised in other countries. Technically the person in question still possesses both citizenships. For example, the U.K. honours renunciation only if done with its competent authorities, and consequently citizens naturalised in the US remain British citizens in the eyes of the British government even though they are required by US authorities to renounce allegiance to any foreign power.