In addition to a valid passport, persons traveling under the VWP must fill out form I-94 W on the plane or ship. This form will be provided by the travel company. It has to be presented to a CBP (Customs and Border Protection) officer at the airport or seaport. Please note that form I-94 W must not be completed and sent in prior to arrival (e.g. in the form of copies from the internet). Only forms handed out by the airline or shipping company may be used.
The information requested on the I-94 W form includes personal data (name, passport number, address etc.) for identification, as well as reasons not to be admitted into the US, such as diseases, criminal activities, criminal records, prior denials to enter the US and general security questions.