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At Birth
Every person is a citizen of some country at birth. Very often children who are born in a country are automatically citizens of that country, this true for the USA. In some nations, Switzerland is an example, you are not granted citizenship by being born in the country's territory. In these countries, your citizenship is considered the same as that of your parents. In many cases, if your parents are each a citizen of a different country, you automatically become a citizen of each of the two countries. In some cases, that dual citizenship only applies until adulthood, at which time you must choose one country or the other.
Marriage
If you marry a citizen of another country, you often become eligible to be a citizen yourself. In the past it was common for this citizenship to be granted automatically. Nowadays, it is rare to receive citizenship automatically via marriage, but spouses are often given permanent resident status and can work more quickly toward citizenship if they choose to do so.
Naturalization
Most countries have some process of naturalization for immigrants. This process usually includes learning about the laws and history of the country, learning the language of the country and then becoming a citizen. In many countries, if the parents of a family become naturalized citizens, their minor children also automatically receive citizenship. U.S. law does not mention dual citizenship and does not require naturalized citizens to give up their citizenship elsewhere. Also, U.S. citizens applying for dual nationality are not required to give up their U.S. citizenship unless the other country requires renunciation.
Legal Obligations
All citizenship carries legal obligations, especially regarding taxation, travel restrictions and military service. This can create difficulties for dual citizens because they are legally obligated in both countries, and sometimes these laws are at odds with each other. A person could find himself having to serve in the army, for example, in both countries if military service is compulsory. A citizen of two countries may also find herself liable for taxes in both countries if the countries don't have some sort of tax treaty. The person is required to obey the laws of both countries, however the country where the dual national is located at the time usually has the strongest claim.
Advantages
Dual citizenship allows a person greater flexibility. Dual citizens have more choices about where to live and to work. They may be allowed to travel under one of their passports to a place prohibited by the other. U.S. citizens must leave and enter the USA on their U.S. passport. If the other country of citizenship has a similar requirement, then they must carry both passports when traveling between those two countries.
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