Anytime you enter or leave the United States, you need to present a valid passport to confirm your identity and citizenship. Most passports are valid for 10 years from the date of issue. While you should renew your passport before the expiration date, you might have to renew far in advance of that date. Many countries require passports to be valid for several months after you return from your trip, so even a passport that is still technically valid may not allow you to enter your destination country.