As of 2009, citizens of Visa Waiver Program countries will be able to travel without applying for an ordinary visa, but they must obtain an ESTA authorization before traveling to the United States.

The Authorization ESTA also defined"Electronic travel authorization system”is a particular concession issued by the Department of Homeland Security which can be requested and obtained by citizens of the countries belonging to the Visa Waiver Program to be authorized to travel to the United States for business reasons, for tourism, or for transit only.

ESTA allows you to stay in the United States for a period of 90 days and has a validity of 2 years from the date of authorisation or until the expiry of the passport, if sooner.
From 6 July 2023, any new ESTA application for Brunei nationals using a Brunei passport will have a maximum validity period of one year. As of 1 August 2023, any new ESTA application for Hungarian nationals using a Hungarian passport will have a maximum validity period of one year.
An unlimited number of trips to the United States are allowed during these two years, but each consecutive stay may not exceed 90 days in duration.

Furthermore, it is essential to specify that the ESTA replaces the green I-94W form that citizens belonging to the VWP were required to fill out in flight before arriving in the United States. In fact, it can be considered a simpler and faster alternative to the non-immigrant visa procedures.