Renunciation of citizenship
Renunciation is the voluntary act of relinquishing one's citizenship or nationality. It is the opposite of naturalization whereby a person voluntarily acquires a citizenship, and distinct from denaturalization, where the loss of citizenship is forced by a state.
Historic practices 
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The common law doctrine of perpetual allegiance denied an individual the right to renounce obligations to his sovereign. The bonds of subjecthood were conceived in principle to be both singular and immutable. These practices held on in varying ways until the late 19th century.
The refusal of many states to recognize expatriation became problematic for the United States, which had a large immigrant population. The War of 1812 was caused partly by Britain's impressment of British-born U.S. citizens into the British Royal Navy. Immigrants to the U.S. were sometimes held to the obligations of their foreign citizenship when they visited their home countries. In response, the U.S. government passed the Expatriation Act of 1868 and concluded various treaties, the Bancroft Treaties, recognizing the right to renounce one's citizenship.
The Universal Declaration of Human Rights lists both the right to leave any country, including ones own (Article 13(2)) and the right to change ones nationality (Article 15(2)), which implicitly recognizes the right to renounce citizenship.
In modern law 
Renunciation of citizenship is particularly relevant in cases of multiple citizenship, given that additional citizenships may be acquired automatically and may be undesirable. Many countries have pragmatic policies that recognize the often arbitrary nature of citizenship claims of other countries and negative consequences, such as loss of security clearance, can mostly be expected only for actively exercising foreign citizenship, for instance by obtaining a foreign passport. People from some countries renounce their citizenship to avoid compulsory military service. However, some people may wish to be free even of the purely theoretical obligations and appearance of dual loyalty that another citizenship implies. In 2006, Boris Johnson, the Mayor of London, prompted by an international travel requirement in the U.S., renounced the US citizenship he had acquired by birth. He said, "What I want is the right not to have an American passport." Another example may be political refugees who would wish to renounce allegiance to the country from which they escaped.
Renunciation law in specific countries 
Each country sets its own policies for formal renunciation of citizenship. There is a common concern that individuals about to relinquish their citizenship do not become a stateless person and many countries require evidence of another citizenship or an official promise to grant citizenship before they release that person from citizenship. Some countries may not allow or do not recognize renunciation of citizenship or establish administrative procedures that are essentially impossible to complete.
Mexico requires renunciation of all other citizenships as a condition of naturalization. Canada and Israel, on the other hand, do not require the renunciation of other citizenships/nationalities as a precondition to running for public office.
Renunciation of citizenship is most straightforward in those countries which recognize and strictly enforce a single citizenship. Thus, voluntary naturalization in another country is considered as "giving up" of one's previous citizenship or implicit renunciation. For practical reasons, such an automatic renunciation cannot officially take place until the authorities of the original country are informed about the naturalization. In Japan, a formal report is required from the renouncing person to be submitted at an embassy. Germany actively investigates whether its citizens living abroad have naturalized there when they apply for a passport; for instance in Canada, German passport applicants have to submit a search of citizenship record. Canada and Australia are signatories to the United Nations Convention on the Reduction of Statelessness, and renunciation is possible only if it will not result in statelessness. Applications for renunciation of citizenship need to be submitted in those countries with a waiting time of several months until approval.
Citizens of Singapore can only renounce Singaporean citizenship if they have acquired citizenship of another country. At the time of renunciation, renunciants must submit their Singaporean passports and National Registration Identity Cards for cancellation. Male Singaporeans generally cannot renounce citizenship until completing national service, though there are exceptions for those who emigrated at a young age. In 2012, a Hong Kong-born man who had acquired Singaporean citizenship by registration while a minor filed a lawsuit regarding this requirement.
About 1,200 Singaporean citizens renounce their citizenship every year, a quarter of them naturalised citizens. Singapore does not permit multiple citizenship for adults. The total number of renunciants from 2001 to 2011 was roughly 10,900. Of these, roughly half were Chinese Singaporeans (who comprise about three-quarters of the population), while the rest were of other races such as Malay or Indians (who comprise about one-quarter of the population).
United States 
The United States requires that an individual go in person to a U.S. embassy or consulate outside the U.S. and sign before a consular officer an oath or affirmation that he intends to renounce his citizenship, although exceptions are legally permitted in times of war and under special circumstances. During the expatriation procedure, the individual must complete several documents and demonstrate in an interview with a consular officer that the renunciation is voluntary and intentional. Depending on the embassy or consulate, the individual is often required to appear in person two times and conduct two separate interviews with consular officers over the course of several months.
Formal confirmation of the loss of U.S. citizenship is provided by the Certificate of Loss of Nationality and is received by the renunciant a number of months later. Renunciation of U.S. citizenship was free until July 2010, at which time a fee of $450 was established. Although many countries require citizenship of another nation before allowing renunciation, the United States does not and an individual may legally renounce U.S. citizenship and become stateless. Nonetheless, the United States Department of State warns renunciants that, unless they already possess a foreign nationality or are assured of acquiring another nationality shortly after completing their renunciation, they would become stateless and without the protection of any government.
In one case, Vincent Cate, an encryption expert living in Anguilla, chose to renounce his US citizenship to avoid the possibility of violating US laws that may prohibit US citizens from "exporting" encryption software.
The United States is the only country in the world which taxes based on citizenship rather than residency, on worldwide income of individuals, in the same manner as residents. Eritrea also taxes the foreign income of its nonresident citizens but at a much lower rate than for residents. Finland, France, Hungary, Italy and Spain tax their nonresident citizens as residents only in limited circumstances. Cuba, North Korea, the Philippines and Saudi Arabia tax residents differently based on their citizenship, but do not tax the foreign income of nonresident citizens. Mexico, the Soviet Union, the Philippines, Vietnam and Myanmar also used to tax the foreign income of nonresident citizens, either in the same manner as residents or at lower rates, but have all abolished this practice.
In 1996, the U.S. changed its immigration law to include a provision to "name and shame" renunciants. The Department of the Treasury became obligated to publish quarterly in the Federal Register the names of those citizens who renounce their citizenship. Only the names are published, but by counting the number of names in each list, media organizations are able to infer the number of renunciants each quarter. The 1996 law included a provision to bar entry to any individual "who officially renounces United States citizenship and who is determined by the Attorney General to have renounced United States citizenship for the purpose of avoiding taxation by the United States." There is no known case of this provision, known as the Reed Amendment, having ever been enforced.
In 2008, Congress enacted the Heroes Earnings Assistance and Relief Act that imposes a penalty—an "exit tax" or expatriation tax—on certain people who give up their U.S. citizenship or long-term permanent residence. Effective June 2008, U.S. citizens who renounce their citizenship are subject under certain circumstances to an expatriation tax, which is meant to extract from the expatriate taxes that would have been paid had he remained a citizen: all property of a covered expatriate is deemed sold for its fair market value on the day before the expatriation date, which usually results in a capital gain, which is taxable income. Eduardo Saverin, a Brazilian-born co-founder of Facebook, renounced his U.S. citizenship just before the company's expected initial public offering; the timing prompted media speculation that the act was motivated by potential U.S. tax obligations.
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