Naturalization (or naturalisation) is the legal act or process by which a non-national of a country acquires the nationality of that country after birth. The definition of naturalization by the International Organization for Migration of the United Nations excludes citizenship that is automatically acquired (e.g. at birth) or is acquired by declaration. Naturalization usually involves an application or a motion and approval by legal authorities. The rules of naturalization vary from country to country but typically include a promise to obey and uphold that country's laws and taking and subscribing to an oath of allegiance, and may specify other requirements such as a minimum legal residency and adequate knowledge of the national dominant language or culture. To counter multiple citizenship, some countries require that applicants for naturalization renounce any other citizenship that they currently hold, but whether this renunciation actually causes loss of original citizenship, as seen by the host country and by the original country, will depend on the laws of the countries involved. Naturalization is politicized due to the reshaping of the electorate of the country.

The massive increase in population flux due to globalization and the sharp increase in the numbers of refugees following World War I created many stateless persons, people who were not citizens of any state. In some rare cases, laws for mass naturalization were passed. As naturalization laws had been designed to cater for the relatively few people who had voluntarily moved from one country to another (expatriates), many western democracies were not ready to naturalize large numbers of people. This included the massive influx of stateless people which followed massive denationalizations and the expulsion of ethnic minorities from newly created nation states in the first part of the 20th century.

Since World War II, the increase in international migrations created a new category of migrants, most of them economic migrants. 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 – jus matrimonii – or by having ancestors who are nationals of that country, in order to reduce the scope of this category. However, in some countries this system still maintains a large part of the immigrant population in an illegal status, albeit with some massive regularizations. Examples include Spain under José Luis Rodríguez Zapatero's government, and Italy under Silvio Berlusconi's government. Some countries allow naturalization due to military service.

Myanmar and Uruguay are currently the only countries in the world that deny immigrants any path to naturalization. Uruguayan legal citizenship has special characteristics. A person who acquires it retains their nationality of origin, which is determined by Uruguayan law to be that of their country of birth and is therefore immutable. Legal citizens acquire political rights but do not acquire Uruguayan nationality as natural citizens do. According to Uruguayan law, those born in Uruguay or whose parents or grandparents are Uruguayan natural citizens are considered to be Uruguayan nationals.

As a result of Uruguay's unusual distinction between citizenship and nationality (it is the only country in the world that recognizes the right to citizenship without being a national), legal citizens have encountered problems with their Uruguayan passports at airports around the world since 2015. This is due to recommendations in the seventh edition of Doc. 9303 of the International Civil Aviation Organization (ICAO), which requires that travel documents issued by participating states include the "Nationality" field. The lack of a naturalization path means that the Nationality field in legal citizens' passports indicates their country of birth, which Uruguay assumes to be their nationality of origin. Many countries do not accept passports issued by a country that declares the holder to be a national of another country. As a consequence, it has severely curtailed legal citizens' exercise of the right to free movement, as their travel abroad is often difficult or downright impossible.

A few rare mass naturalization processes have been implemented by nation states. In 1891, Brazil granted naturalization to all aliens living in the country. In accordance with the 1923 population exchange agreement between Greece and Turkey following the Armistice of Mudanya and end of World War in Anatolia, Greece rapidly naturalized all Greek, Roman or Orthodox Christian people fled from Turkey after the defeat of Greek Army in Turkey. Reciprocally, the Republic of Turkey naturalized all Turk or Muslim refugees and deportees (Bosniaks, Pomaks, Macedons, Muslim Greeks) from Balkans (after Balkan Wars), Greece and other former regions of Ottoman Empire during redemption period. Another massive naturalization process was in favor of Armenian people coming from Anatolia, who went to Syria, Lebanon or other territorries of former Ottoman Empire. In 1980s and 1990s Turks and Muslims living in Bulgaria were forced to leave Bulgaria in a massive campaign. Republic of Turkey accepted almost all refugees and deportees from Bulgaria and naturalized them. A significant refugee from that era was the athlete Naim Süleymanoğlu.

Canada instituted a mass naturalization by Act of Parliament with the enactment of the Canadian Citizenship Act 1946.

After annexation of the territories east of the Curzon line by the Soviet Union in 1945, Soviets naturalized en masse all the inhabitants of those territories—including ethnic Poles, as well as its other citizens who had been deported into the Soviet Union, mainly to Kazakhstan. Later on^[when?], Germany granted to the ethnic German population in Russia and Kazakhstan full citizenship rights. Poland has a limited repatriation program in place.