The Immigration and Naturalization Act sets forth the legal requirements for acquiring and losing citizenship of the United States.
The requirements become more explicit since the ratification of the Fourteenth Amendment to the Constitution, with the most recent changes made in 2001.
Rights of Citizens
U.S. citizens have the right to participate in the political system of the United States (with most U.S. states having restrictions for felons, and federal restrictions on naturalized persons), are represented and protected abroad by the United States (through U.S. embassies and consulates), and are allowed to reside in the United States, and certain territories, without any immigration requirements.
Acquisition of Citizenship
Citizenship of U.S.A. can be obtained by:
Most United States citizens are natural-born citizens, meaning they have been citizens since birth by virtue of having been born in the United States or born to United States citizens overseas.
Birth within the United States
Children born in the United States (including not only the 50 states and the District of Columbia, but also, in most cases, US Territories, such as Puerto Rico, Guam, the Northern Mariana Islands, the U.S. Virgin Islands, and the Panama Canal Zone before it was returned to Panama, in addition to many current states which were territories at the time of the birth of some individuals now living, e.g. Arizona), are U.S. citizens at birth (unless born to foreign diplomatic staff), regardless of the citizenship or nationality of the parents. This has become controversial, as some non-resident parents enter the United States to give birth, so that their children, often called anchor babies, will be U.S. citizens. A birth certificate is considered evidence of citizenship.