Qualifications for natural born American citizenship:
- Birth on American soil, including outlying possessions, embassies and overseas military bases
This is the commonest way of obtaining American citizenship. (Note that the parents do not need to be American citizens. This causes regular headaches for the INS, which must choose between deporting the citizen children of illegal aliens and letting those aliens stay for the sake of their kids.)
- Birth abroad, to two American citizen parents
At least one of the parents must have been resident in the US or one of its outlying possessions prior to the birth. There is no minimum time for the parent's residency.
- Birth abroad to one American citizen parent and one non-American parent
- Children born between December 24, 1952 and November 13, 1986:
The American citizen parent must have been physically present in the United States for ten years prior to the birth, at least five years of which must be after his/her fourteenth birthday.
- Children born on or after November 14, 1986:
The American citizen parent must have been physically present in the United States or one of its outlying territories for five years before the birth, at least two of which were after the age of fourteen.
- Birth abroad to an American citizen mother, out of wedlock
The mother must have been physically present in the United States for a continuous period of at least one year before the birth of the child.
- Birth abroad to an American citizen father, out of wedlock
In addition to the requirements in section 3 above, the father must sign a sworn statement agreeing to provide financial support for the child until s/he reaches the age of 18 years, and acknowledge paternity. If the father does not acknowledge paternity, the child must be legitimated under local law, or a competent court must establish paternity.
Minors (under 18) who do not meet the citizenship requirements above, but who have at least one American grandparent who meets the physical presence requirements, can apply for expeditious naturalization. Rules also exist for adopted children to obtain US citizenship. There is an application process for adults who wish to become naturalized citizens of the United States. However, only natural born citizens (fulfilling one of the requirements above) can become President or Vice President, according to Article II of The Constitution of the United States of America. As if you'd want to, anyway...
See also: naturalized citizen
This information is based on my understanding and experience with the system, and may be superseded by later legislation. IANAL.
Source: American Embassy factsheet accompanying Form FS-579, Application for Consular Report of Birth