Currently, Title 8 of the U.S. Code fills in those gaps. Section 1401 defines the following as people who are “citizens of the United States at birth:”

Anyone born inside the United States
Any Indian or Eskimo born in the United States, provided being a citizen of the U.S. does not impair the person’s status as a citizen of the tribe
Any one born outside the United States, both of whose parents are citizens of the U.S., as long as one parent has lived in the U.S.
Any one born outside the United States, if one parent is a citizen and lived in the U.S. for at least one year and the other parent is a U.S. national
Any one born in a U.S. possession, if one parent is a citizen and lived in the U.S. for at least one year
Any one found in the U.S. under the age of five, whose parentage cannot be determined, as long as proof of non-citizenship is not provided by age 21
Any one born outside the United States, if one parent is an alien and as long as the other parent is a citizen of the U.S. who lived in the U.S. for at least five years (with military and diplomatic service included in this time)
A final, historical condition: a person born before 5/24/1934 of an alien father and a U.S. citizen mother who has lived in the U.S.
Anyone falling into these categories is considered natural-born, and is eligible to run for President or Vice President. These provisions allow the children of military families to be considered natural-born, for example.

Separate sections handle territories that the United States has acquired over time, such as Puerto Rico (8 USC 1402), Alaska (8 USC 1404), Hawaii (8 USC 1405), the U.S. Virgin Islands (8 USC 1406), and Guam (8 USC 1407). Each of these sections confer citizenship on persons living in these territories as of a certain date, and usually confer natural-born status on persons born in those territories after that date. For example, for Puerto Rico, all persons born in Puerto Rico between April 11, 1899, and January 12, 1941, are automatically conferred citizenship as of the date the law was signed by the President (June 27, 1952). Additionally, all persons born in Puerto Rico on or after January 13, 1941, are natural-born citizens of the United States. Note that because of when the law was passed, for some, the natural-born status was retroactive.

The law contains one other section of historical note, concerning the Panama Canal Zone and the nation of Panama. In 8 USC 1403, the law states that anyone born in the Canal Zone or in Panama itself, on or after February 26, 1904, to a mother and/or father who is a United States citizen, was “declared” to be a United States citizen. Note that the terms “natural-born” or “citizen at birth” are missing from this section.

In 2008, when Arizona Senator John McCain ran for president on the Republican ticket, some theorized that because McCain was born in the Canal Zone, he was not actually qualified to be president. However, it should be noted that section 1403 was written to apply to a small group of people to whom section 1401 did not apply. McCain is a natural-born citizen under 8 USC 1401(c): “a person born outside of the United States and its outlying possessions of parents both of whom are citizens of the United States and one of whom has had a residence in the United States or one of its outlying possessions, prior to the birth of such person.”

Not that I’m any fan of the “maverick”= but right is right.