Many attendees from outside the US will require a B-1 Visitor Visa to enter the United States. See here for authoritative visa requirements from the US Department of State.
Attendees from the 38 countries in the Visa Waiver Program must still register with the Electronic Systems for Travel Authorization (ESTA) and possess an e-passport. Some with older pre-e-passports have found that it is easier to get a new passport than apply for a regular visa.
Most visa applications typically require an interview. The wait time to receive an interview appointment at a U.S. Embassy or Consulate could be over 16 days. Please see the US Department of State's page for estimates of visa and processing wait time at different consulates. Please schedule your appointment as early as possible.
Visa Invitation Letters
If you require a letter of invitation, please indicate this when registering for the conference. Unless hard copies are specifically requested, letters will be sent via e-mail. Hard copies can be sent by post at no charge, or express delivery for an extra charge. No letters will be sent until we have verified that registration fees are paid in full.