Travel authorisation mandatory when going to the USA

As from 12 January 2009, all nationals of countries belonging to the Visa Waiver Program (such as Belgian nationals), will be required to obtain an electronic travel authorisation prior to initiating travel to the United States.
This authorisation is obtained online (www.cbp.gov/esta) through the Electronic System for Travel Authorisation (ESTA).

This formality is not a visa application and is totally free of charges.

If you are not in possession of a travel authorisation, then, in principal, you are not allowed to fly to the USA. The authorisation is valid for 2 years, provided that your situation has not changed in the meantime and that your passport is not expired. When the application for a US travel authorisation is denied, you are obliged to apply for a visa.