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U.S. Department of Homeland Security (DHS) has announced the introduction of ESTA, which established a new online system that is part of the Visa Waiver Program (VWP).

ESTA: Electronic System for Travel Authorization
Currently, passengers who travel to the U.S. under VWP have to fill out I-94w form. After ESTA is implemented, passengers who travel to the U.S. using VWP have to get pre-travel authorization through ESTA.

Passengers who plan to travel to the United States on/after January 12th, 2009 under VWP(VWP countries,
please refer to here). must get pre-travel authorization though ESTA by themselves before check-in. Please get pre-travel authorization by yourself before you come to airport, as passengers without pre-travel authorization can not be checked-in.

Go to ESTA Website
Step-by-step guide to ESTA (English PDF)

For further details,

U.S. Department of Homeland Security