If you were born in the United States, you are automatically a US citizen. Even if your parents were not citizens, or they weren't in the country lawfully - even if you left the US the day you were born and never returned - you're a US citizen if you were born on US soil.
What qualifies as "US soil"? Obviously, it includes all 50 US states. It also includes Puerto Rico, Guam, the US Virgin Islands, and the Commonwealth of the Northern Mariana Islands. If you were born in any of these locations, you are a US citizen.
There is only one exception to this rule. The children of foreign diplomats who are born on US soil do not automatically acquire US citizenship.
If both your parents were US citizens, but you were born outside the country, you are probably a US citizen. The answer usually depends on whether your parents were married at the time you were born. If they were married, you are a citizen so long as at least one of your parents lived in the US at some point before you were born. If they were not married, you must show that you have a valid citizenship claim through one of your parents.
If only one of your parents was a citizen of the United States, things get very complicated. You should speak with an experienced attorney to discuss specifics, but in general, you are a citizen if:
If one of your parents became a US citizen after you were born, you may acquire citizenship through that parent. The rules are different depending on when you were born, but most people have to show that they were less than 18 years old when their parent became a citizen,and that they were living in the US as a lawful permanent resident at the time.
If you were adopted by a US citizen, you may have automatically acquired citizenship through your adoptive parent. Generally, an adoptee born after 1982 is a citizen if they were admitted to the US as a lawful permanent resident and lived in the US with the adoptive US citizen parent.
These articles are part of our ongoing "Immigration 101" series, in which we break down topics in US immigration law.**