A visitor, or temporary, visa is required of all visitors seeking to enter the USA with the exception of nationals of Canada and the 37 countries that participate in the Visa Waiver Program. Travel to the United States under the Visa Waiver Program Requires ESTA (Electronic System for Travel Authorization) Approval. It's easy to apply for one online but be sure to have it completed at least 72 hours before your trip or you could be denied entry.