法律上的'''归化'''，是指某个人在出生国籍以外自愿、主动取得其他国家国籍的行为。一般是居住在国外的人，依据所居国的法律规定取得新国籍。归化的反义词是剥夺国籍（denaturalization）。

==基本要求==
* 在归化国合法居住规定的时间
* 申请人保证遵守国家法律
* 在有些国家，申请人要效忠宣誓
* 部份國家要求申請人通過語言（如法國）及本國文化、本國法律等項目的考試。
* 有些国家的条件是归化者放弃其他国籍，以避免雙重國籍现象的产生。但到底保留原来国籍与否，要依据原来国家和入籍国家双方的法律决定。

一个不寻常的归化情况是以色列。1950年通过的《犹太人回归法》（Law of Return,
&#8206;）规定：从血统、婚姻或宗教方面符合其定义的犹太人有权申请定居以色列并立即获得以色列国籍。

==大规模归化==
例子：
*1922年，希腊给予了土耳其的希腊裔难民大规模归化待遇。
* 2001-2002年，阿根廷发生了严重经济危机。西班牙和意大利分别允许能证明自己祖父辈以内是本国血统的阿根廷公民申请归化。

==参见==
*国籍

A judge swears in a new citizen. [[New York, 1910|thumb]]

The origin of the term "naturalization" is that it gives to a resident alien almost all of the rights held by a natural-born citizen.

In general, basic requirements for naturalization are that the applicant hold a legal status as a full-time resident for a minimum period of time and that the applicant promise to obey and uphold that country's laws, to which an oath or pledge of allegiance is sometimes added. Some countries also require that a naturalized national must renounce any other citizenship that he currently holds, forbidding dual citizenship, but whether this renunciation actually causes loss of the person's original citizenship will again depend on the laws of the countries involved.

Nationality is traditionally either based on ''denizens. In some rare cases, procedures of mass naturalization were passed (Greece in 1922, Armenian refugees or, more recently, Argentine people escaping the economic crisis). As naturalization laws had been created to deal with the rare case of people separated from their Russians who had escaped the 1917 Spanish refugees. As Hannah Arendt pointed out, internment camps became the "only nation" of such stateless people, since they were often considered "undesirable" and were stuck in an illegal situation (their country had expelled them or deprived them of their nationality, while they hadn't been naturalized, thus living in a judicial no man's land). 

After international migrations created a new category of refugees, most of them economic refugees. For economic, political, humanitarian and pragmatic reasons, many states passed laws allowing a person to acquire their citizenship after birth (such as by marriage to a national or by having José Luis Zapatero's government and in Italy by Berlusconi's government).

==Different naturalization laws==
=== In Finland ===
Finland became independent on December 6, 1917. The old constitution, dating back to Swedish rule, required all Finnish citizens to be of Evangelical Lutheran faith. Both Jews and Muslims started to apply for Finnish citizenship in 1918. Muslims, however, were accepted only after the Constitution of Finland was modified and general freedom of religion was declared by 1919.

=== In the United Kingdom ===

There had always been a distinction in English law between the subjects of the monarch and aliens: the monarch's subjects owed him (or her) allegiance, and included those born in his dominions (natural-born subjects) and those who later gave him their allegiance (naturalized subjects).

The modern requirements for naturalisation as a British citizen depend on whether one is married to a British citizen or not. 

For those married to a British citizen the applicant must:
* hold  indefinite leave to remain in the UK (or an equivalent such as Right of Abode or Irish citizenship)
* have lived legally in the UK for three years
* been outside of the UK no more than 90 days during the one-year period prior to filing the application.
* show sufficient knowledge of life in the UK, either by passing the Life in the United Kingdom test or by attending combined English language and citizenship classes.  Proof of this must be supplied with one's application for naturalisation.  Those aged 65 or over may be able to claim exemption.
* meet specified English, Welsh or Scottish Gaelic language competence standards.  Those who pass the ''Life in the UK test'' are deemed to meet English language requirements. 

For those not married to a British citizen the requirements are:
* at least five years legal residence in the UK
* been outside of the UK no more than 90 days during the one-year period prior to filing the application.
* indefinite leave to remain or equivalent must have been held for 12 months
* the applicant must intend to continue to live in the UK or work overseas for the UK government or a British corporation or association. 
* the same language and knowledge of life in the UK standards apply as for those married to British citizens

All applicants for naturalisation must be of "good character". Naturalisation is at the discretion of the Home Secretary but is normally granted if the requirements are met.

=== In the United States ===

In the United States of America, naturalization is mentioned in the Constitution. 

thumbnail|right|350px|A Naturalization Certificate from 1911
Congress is given the power to prescribe a uniform rule of naturalization, which was administered by state courts. There was some confusion about which courts could naturalize; the final ruling was that it could be done by any "court of record having common-law jurisdiction and a clerk (prothonotary) and seal."

The Constitution also mentions 'natural born citizen.' The first naturalization Act (drafted by Thomas Jefferson) used the phrases 'natural born' and 'native born' interchangeably. To be 'naturalized' therefore means to become as if "natural born" -- i.e. a citizen.

There is an interesting loophole here in that the Constitution does not mandate race-neutral naturalization. Until 1952, the Naturalization Acts written by Congress still allowed only white persons to become naturalized as citizens (except for two years in the 1870s which the Supreme Court declared to be a mistake).

Naturalization is also mentioned in the Fourteenth Amendment. Before that Amendment, individual states set their own standards for citizenship. The Amendment states that "all persons born or naturalized in the United States and subject to the jurisdiction thereof shall be citizens of the United States and of the State in which they reside."

Note also that the Amendment is ambiguous on the issue of singular or plural United States. In the early days the phrase 'United States' was used as a singular or a plural according to the meaning. After the Civil War, it was generally always a singular. The Amendment does not say 'its jurisdiction' or 'their jurisdiction' but 'the jurisdiction thereof'.

The Federalists and extended the residency requirement from five to fourteen years. It  specifically targeted Irish and French immigrants who were involved in anti-Federalist politics. It was repealed in 1802.

An 1862 law allowed honorably discharged Army veterans of any war to petition for naturalization, without having filed a declaration of intent, after only one year of residence in the United States. An 1894 law extended the same privilege to honorably discharged 5-year veterans of the Navy or Marine Corps. Over 192,000 aliens were naturalized between May 9, 1918, and June 30, 1919, under an act of May 9, 1918. Laws enacted in 1919, 1926, 1940, and 1952 continued preferential treatment provisions for veterans. <ref>Schulze, Lorine McGinnis (2003) http://www.naturalizationrecords.com/usa/ Retrieved April 23, 2005</ref>

Passage of the Fourteenth Amendment meant that, in theory, all persons born in the U.S. are citizens regardless of race. Citizenship by birth in the United States, however, was not initially granted to Asians until 1898, when the Supreme Court held that the Fourteenth Amendment did apply to Asians born in the United States in United States v. Wong Kim Ark.

The enabling legislation for the naturalization aspects of the Fourteenth Amendment was the 1875 Page Act, which allowed naturalization of 'aliens of African nativity and to persons of African descent,' but is silent about other races. 

The 1882 Chinese Exclusion Act banned Chinese workers and specifically barred them from naturalization. The Immigration Act of 1917, (Barred Zone Act) extended those restrictions to almost all Asians. 

The 1922 Cable Act specified that women marrying aliens ineligible for naturalization lose their US citizenship. At the time, all Asians were ineligible for naturalization. The Immigration Act of 1924 barred entry of all those ineligible for naturalization, which again meant non-Filipino Asians.

Following the Philippine residents were classified as US nationals. But the 1934 Tydings-McDuffie Act, or ''Philippine Independence Act'', reclassified Filipinos as aliens, and set a quota of 50 immigrants per year, and otherwise applying the Immigration Act of 1924 to them. 

Asians were first permitted naturalization by the 1943 Magnuson Act, which repealed the Chinese Exclusion Act. India and the Philippines were allowed 100 annual immigrants under the 1946 Filipino Naturalization Act. The War Brides Act of 1945 permitted soldiers to bring back their foreign wives. 

The 1952 Immigration and Nationality Act (better known as the McCarran-Walter Act), lifted racial restrictions, but kept the quotas in place. The Immigration Act of 1965 finally allowed Asians and all persons from all nations be given equal access to immigration and naturalization. 

Illegal immigration became a major issue in the US at the end of the 20th century. The Immigration Reform and Control Act of 1986, while tightening border controls, also provided the opportunity of naturalization for illegal aliens who had been in the country for at least four years.

The adopted internationally. A child under age 18 who is adopted by at least one U.S. citizen parent, and is in the custody of the citizen parent(s), is now automatically naturalized once admitted to the United States as an immigrant.