In law, naturalization is the act whereby a person voluntarily and actively acquires a nationality which is not his or her nationality at birth. Naturalization is most associated with economic migrants or refugees who have immigrated to a country and resided there as an alien, and who have voluntarily chosen to become a citizen of that country after meeting specific requirements. Denaturalization is the reverse of naturalization, when a state deprives one of its citizens of his or her citizenship. After World War I, many European countries, including democracies, passed denaturalization laws, of which the 1935 Nuremberg Laws remained the most famous.
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 nationalities that he currently holds, forbidding dual citizenship, but whether this renunciation actually causes loss of the person's original nationalities will again depend on the laws of the countries involved.