Visa Waiver Program travelers are required to apply for and receive an approved travel authorization via ESTA to board a plane or vessel bound for the United States. (A travel authorization via ESTA is not a guarantee of admission into the United States. Travelers must establish their eligibility for the program during inspection by an officer at a port of entry.)
In most instances, ESTA applicants will receive real-time notification on the status of their application to travel to the United States. Applicants denied an ESTA will be advised to apply for a visa at the nearest U.S. consulate or embassy.
For more information, and to apply for a travel authorization, see ESTA.