Proof of citizenship is what an individual must show to travel abroad and gain reentry to the United States. There are a variety of different ways that an individual may use to show that they are in fact a citizen of that given country. If an individual has to show proof of citizenship, it can be beneficial to know what qualifies in advance to prevent any problems. It is likely that when an individual is crossing the border or trying to gain entry into their home country, that they will need to show some sort of proof of citizenship. Usually a passport or birth certificate will qualify as a valid proof of citizenship.