It is imperative that authors and delegates allow sufficient time to
Information on United States visas can be found here and
here. It is advisable to begin the application process as early as possible to avoid last minute problems. The time to obtain a visa can vary widely and has taken over three months for some individuals in the past. An interview at a consulate may also be required. Authors who wait until notification of the acceptance of their papers before applying for visas risk not receiving visas in time to present their work.
The US Department of Homeland Security has implementated the Electronic System for Travel Authorization (ESTA).
ESTA is a new fully automated, electronic system for screening
passengers before they begin travel to the United States under the Visa Waiver Program.
ESTA applications may be submitted at any time prior to travel to the
United States, and VWP travelers are encouraged to apply for
authorization as soon as they begin to plan a trip to the United
States. ESTA has become mandatory for all travelers from current Visa Waiver Program member countries. Frequently asked questions available here.