ESTA, or the Electronic System for Travel Authorization, is an automated system used to determine the eligibility of visitors to travel to the United States under the Visa Waiver Program (VWP). It requires travellers from VWP countries to complete an online application, which assesses their biographic and eligibility information. ESTA approval does not guarantee entry into the United States; it simply authorises a traveller to board a carrier bound for the U.S. Applications should be submitted at least 72 hours before travel, and once approved, an ESTA is valid for two years or until the traveller's passport expires, whichever comes first. Travellers are advised to print a copy of their ESTA approval or have a digital copy ready to show at the airline check-in and U.S. immigration.