There are many requirements for visitors to enter the United States.
Citizens of countries that participate in a Visa Waiver Program (see https://travel.state.gov/content/travel/en/us-visas/tourism-visit/visa-waiver-program.html for a complete list) with the United States generally are only required to register through the Electronic System for Travel Authorization (ESTA at https://www.cbp.gov/travel/international-visitors/esta) prior to entry into the US.
Citizens of all other countries must obtain a visa prior to arriving at any U.S. port of entry. The U.S. State Department (https://travel.state.gov/content/travel/en/us-visas.html) website contains up to date information on entry requirements for citizens of countries that require Visas for entry.
Visa applications are currently subject to a greater degree of scrutiny than in the past. Due to the lengthiness of some visa processes, we generally advise that the potential conference attendee begin and submit their visa application not less than 90 days prior to the Conference. For approval to enter the United States, some conference attendees will need to electronically complete DS160 form and participate in a face-to-face interview at an Embassy or local Consulate to obtain a B-1 visa.
Many embassies request the prospective conference attendee to present an invitation letter from the ICOPS 2023 organizing committee to verify that the conference has accepted the applicant's abstract and paid their registration fee in full.
Some applicants may be required to appear in person for an interview as part of the visa process. In these cases, applicants should anticipate delays in getting an appointment for the interview and to expect long processing times. Therefore, we advise that visa applicants apply no later than 3 months prior to the conference.