Most travelers to the United States must hold a valid visa and a passport that is valid six months longer than the intended visit. Visitors planning to visit or attend a meeting will most likely apply for a B-1 visa. Visa applicants are advised to apply as soon as they decide to travel to the United States and at least 3 to 4 months in advance. All Visa Waiver Program (VWP) travelers will be required to have a machine-readable passport (MRP) in order to enter the United States without a visa. As a standard part of the visa process, the State Department now requires that consular officers interview almost every applicant. Some consulates may have a long wait for an interview so applicants should contact the consulate to schedule an interview as early as possible.
Contact the nearest U.S. consulate or embassy for details on visa application procedures at that post.

ESTA - Latest information for Visa Waiver Program (VWP) travellers
ESTA is a fully-automated, internet-based electronic system for screening Visa Waiver Program (VWP) passengers before they begin travel to the United States.

On August 1, 2008, ESTA was introduced on a voluntary basis via internet for citizens and eligible nationals of Visa Waiver Program (VWP) countries to apply for advance authorization to travel to the United States under the VWP. Effective January 12, 2009, ESTA will become mandatory and all VWP travelers will be required to obtain an electronic travel authorization prior to boarding a carrier to travel by air or sea to the U.S. under the VWP. ESTA is also available in Dutch, French and English. Click here for information regarding the Electronic System for Travel Authorization (ESTA).

More information can be found at:

U.S. Department of State site for Temporary Visitors to the U.S.
http://travel.state.gov/visa/temp/temp_1305.html

Information on applying for a U.S. nonimmigrant visa
http://travel.state.gov/visa/tempvisitors.html

The National Academies International Visitors Office
http://www7.nationalacademies.org/visas/Traveling_to_US.html

Homeland Security
http://www.dhs.gov/xtrvlsec/crossingborders/

An official letter of invitation can be sent, if needed, to international participants for use in obtaining visas. Please provide your name, title, affiliation, complete mailing address and email address, phone and fax numbers and submit to the INCOSE Conference Manager.