Passports tend to expire every 10 years or so. However, even if a passport is expired, it isn't entirely useless. While you can use your old passport to apply for a new one, you can also use it as proof of your U.S. citizenship, even after it has expired. This means that you can use it like regular identification, such as a driver's license, which can allow you to board domestic flights.
Proof of Citizenship
According to guidelines issued by the U.S. Department of Homeland Security, an expired passport issued by the U.S. government is still considered to be a valid proof of citizenship. This is because the person who received the passport was at one point a citizen, a status they have presumably not lost. However, a canceled passport is not considered a valid form of identification, as the person may have lost her citizenship.
When you board a domestic flight in the United States, you are required to provide a valid form of photo identification that proves you are the person listed on the ticket. In addition to a driver's license, airlines also accept several other forms of I.D., including passports, both current and expired. So, you can travel in the United States with an expired U.S. passport.
However, if your passport is expired, you cannot travel internationally. While a person used to be able to enter Mexico and Canada with just a driver's license, now you are required to have a valid passport to make the trip. If you try to travel internationally on an expired passport, you will be stopped when being checked by a customs agent and sent back to the United States. Foreign visitors to the United States must also have a current passport.
Renewing the Passport
If you have an expired passport, you can usually renew it easily, by sending it in the U.S. State Department, along with the required forms. In several weeks, you should receive a new passport. If you need your passport more quickly, you can also pay more money to have the process expedited. A valid passport serves as proof of citizenship and allows you to board international flights -- although you may also need a visa to travel to certain countries.