Your passport (or birth or naturalization certificate) is your positive proof of national identity; without it, your status in any foreign country is in doubt. Don’t leave home without one. In fact, U.S. Immigration rules require that all U.S. Citizens must have a valid passport in order to re-enter the U.S. United States citizens may obtain passports (allow four to six weeks) at local post offices. For-fee private passport agencies can speed this process and get you a passport within a week, maybe less.