You may obtain United States citizenship either through birth or through naturalization.  In fact, some people unknowingly are already United States citizens based on their birth to a United States citizen parent or parents, depending on various circumstances.

Citizenship through birth in the United States

Under the 14th Amendment of the United States constitution, all persons born within the territory of the United States are citizens.  This rule, based on the latin phrase "jus soli", grants U.S. citizenship in almost all cases to individuals born within the borders of the U.S.  Some exceptions include the children of embassy personnel and representatives to the United Nations, depending on the details of the case.  The borders of the United States includes all 50 states, Puerto Rico, Guam, and the U.S. Virgin Islands.  In addition, there are other circumstances where one's birth may have occurred within the borders of the U.S.

Birth outside the United States to United States citizen parents

Although not provided under the Constitution, current U.S. law provides for an individual to obtain citizenship through his/her parents status as U.S. citizens at the time of the individuals birth in some circumstances.  This method of obtaining U.S. citizenship is based on an individual's bloodline and is known by the latin phrase "jus sanguinis".  Generally speaking, an individual born outside the United States may be a United States citizen at birth if either or both of his or her parents were United States citizens at birth.  However, there are many variables to take into account in determining whether or not an individual has been granted U.S. citizenship through his or her parents at birth.  Some of the variables to take into account are the individual's date of birth, whether one or both parents were United States citizens at the individual's birth, and the prior United States residency status of the U.S. parent/s before the individual's birth. 

Naturalization

Unlike the citizenship processes discussed above, naturalization is the process of obtaining citizenship after birth.  Some of the requirements for an applicant to obtain United States citizenship through naturalization are as follows:

Derivation of United States citizenship through naturalization of a parent

Under certain circumstances, an individual may automatically obtain United States citizenship when one or both parents become naturalized United States citizens.  This method of obtaining U.S. citizenship is automatic and is often referred to as "automatic naturalization" since the benefiting child does not have to participate in any sort of naturalization ceremony. However, the rules vary for derivation of United States citizenship through the naturalization of one's parents.  When evaluating whether or not "automatic naturalization" occurred, one must take into consideration various circumstances, such as the time period of when the parents were naturalized and the child's age and marital status at the time of the parents' naturalization.  All circumstances and aspects of the situation need to be carefully evaluated. 

Obtaining proof of United States citizenship

In order to establish a legitimate claim to United States citizenship, you must apply for and receive a document confirming your status as a U.S. citizen.  There are several types of documents which will be recognized as proof of U.S. citizenship.  They are as follows:  U.S. passport; certificate of citizenship; or certificate of consular registration at birth.