As from January 12, 2009, the US authorities require passengers eligible to travel under the Visa Waiver Program (see above table) to apply for an Electronic System for Travel Authorization (ESTA) online before departure. In most cases the application is handled immediately, and you will receive an instant answer online.
It is the responsibility of the passenger to secure that an ESTA has been approved prior to departure.  The US authorities recommend that an application is done at the latest 72 hours before departure.
An approved ESTA will be valid 2 years and allows for multiple visits to the USA.  In connection with check-in, the airline will check that a passenger has an approved ESTA. During an interim period, the US authorities require an ESTA approval and that the green card  (I-94W) is filled in prior to entering the US. The green card is handed out at check-in and/or onboard the aircraft.

Note! If you have a B1 or B2 visa, you do not need to apply for an ESTA.

When traveling to the United States of America you are required to take following actions to be eligible to enter the country.

Citizens of countries that are member of the Visa Waiver Program

If you do not hold a machine readable passport or if your planned stay will exceed 90 days you must obtain a US visa prior to departure at an US embassy.

Please see the link on our Travelers Guide page to apply for your ESTA now.

Citizens of countries that are not member of the Visa Waiver Program

Contact the US embassy in your country to apply for a visa, the embassy will provide you with all further information needed.