When flying out of the United States to another country, passengers need to show valid passports in order to board the plane, and again at the destination airport. But you most likely won't need a passport just to make the booking – it usually doesn't come into play in the ticket-buying process until it's time for check in, at which point, you'll need to show your passport. Make sure the passport is valid for a certain period of time after the date you plan to return from your trip as required by the country you're visiting.

Purchasing a Ticket

When you sit down to make the big ticket purchase, don't worry if you don't have your passport on hand. Most airlines and third-party travel sites don't require you to input a passport number at the time of booking. At that point, the only information you'll need is contact, payment and billing information.

When to Have Your Passport Ready

Still, when flying internationally, you will need your passport eventually. Most countries require visitors to present a passport that's valid for a significant period of time – such as three or six months – beyond the planned date of return. In most cases, you'll get down to the wire on presenting passport information once it's time to check in for your flight.

International flight check-in times vary depending on the airline, but most open online within 24 to 48 hours of the flight's departure time and close at least 90 minutes before the plane takes off. If you're checking in at the airport, you might be able to do so later, but you'll need to bring your passport to the check-in desk. Not all countries require travelers to present their passports upon departing the country, but the United States is one that does, so you'll have to show your passport before you board your departing flight. All countries require passports for entry, so you'll need to present it again to a border patrol officer once the flight lands.

If You Don't Have a Passport

Make sure you have a valid passport in time to check in to your international flight. Applying for a passport can take as long as several weeks, so get started on the process as soon as you can. Passport processing is estimated to take between four and six weeks, but that's no guarantee, so if you don't have a valid passport, start the application process as soon as you decide to leave the country.

If you're in a rush to get your passport, you can pay an extra fee, about $60, to have it expedited. A routinely expedited passport will arrive in less than four to six weeks. If you're traveling within two weeks, you can make an appointment at a passport agency or center to make sure you receive your passport in time. If you're in a life or death emergency and need a passport immediately, that's possible, as well – but you'll have to prove a serious illness, injury or death in your immediate family that requires travel outside of the United States within three business days.