Passports are definitive proof of citizenship of a particular nationality.
This is the single most important document you can have for travel anywhere in the world. Many countries do not require visas in order to travel to them, however every country requires a passport. We recommend that you have a passport to prove your citizenship if you plan to travel anywhere outside of the United States or your country of residence, even if you are going on a cruise somewhere in the Caribbean. Your U.S. Passport will help you in case of emergency or if you are in need of assistance outside of the United States. A birth certificate or voter registration card will not be of much help.