Most travelers to the United States must hold a valid visa and a passport valid for at least six months longer than the intended visit. If you do need a visa, please understand that the visa application process may take several months. An interview appointment is required for a visa application at all embassies and consulates. The wait time for this appointment varies, but may be as long as three months, with processing taking an additional month or more. As soon as you decide to attend the meeting, you should begin the process of applying for a visa. Should you need an invitation letter, please understand that you either need to have an accepted abstract to the meeting or must be registered and paid in full. Should your visa application not be accepted, we will be happy to refund your registration fee. Please email email@example.com to request a letter of invitation once you've registered or received your abstract acceptance notification.
The United States has made some important changes to the US-VISIT program, which was instituted in 2004. The program now requires most foreign visitors to have a set of complete fingerprints taken along with a digital photograph to verify identity at consulates as well as the port-of-entry. Most Canadians are currently exempt from US-VISIT requirements. A departure confirmation program, as part of US-VISIT, is also now currently in place. This program applies to all visitors, including those from visa waiver countries.
You can get more information regarding visas to the United States here: https://travel.state.gov/content/visas/en/visit/visitor.html
and information regarding the wait time for your particular embassy or consulate here: https://travel.state.gov/content/visas/en.html