If you are not a resident of the United States, you may need a visa to enter. If you already have a visa and need to enter the US, make sure it will not expire before you use it to apply for admission (entry) at the port. Citizens of the following 27 countries do not need a visa to enter the US for tourism or business for stays of 90 days or less :
 Providing you meet the visa waiver criteria:
a) You must have a machine-readable passport (this means it should have a line of chevrons (e.g., <<< ) valid for six months past your expected stay in the U.S.
b) Any passport issued after October 26, 2005 must include a digital photo.
c) You must have a ticket going out of the US. I.e. only a one-way ticket into the US is not sufficient.
Information for Canadians
Canadian citizens generally do not need either visa or passport to enter the US. All travelers should bring evidence of their identity (e.g., a government photo-id) and citizenship (e.g., a passport; or a birth or citizenship certificate). Fiancees of US citizens, or spouses of US permanent residents, may need a visa even if they are Canadian citizens — especially if they are coming to the US to await final immigration status. For details, see usembassycanada.gov.
Getting a visa
To get a visa, contact the nearest US Embassy, Consulate, or other authorized institute to find out how long the process will take (generally anywhere from 3 to 45 days). A list of U.S. Embassies can be found at travel.state.gov. Approximate wait times can be found here. More information can also be found at http://www.unitedstatesvisas.gov.
Some guidelines for painless visa processing:
Ask us for a letter of invitation during registration (it's a checkbox on the form).
Start applying for a visa early, ideally 4+ months in advance. The basic visa process will not take this long, but you will want time to resubmit the application if necessary, and to buy your tickets after the visa has been granted.
Prepare for your visa interview/application. You should have
Your entire travel itinerary, from when you leave your country to when you return
Note that your travel plans depend upon early approval of the visa application
Your invitation, and printed information about the conference
If you are getting financial support to attend the conference, make sure you have printed documentation of this as well.
Proof of association with Wikimania (information about you as a researcher, wiki-developer, Wikipedian, &c.)
Evidence that you will return home -- that is, of "binding or sufficient" ties to your home country (normally your country of residence). Useful examples include:
evidence of family ties in your home country
evidence of property ownership
evidence/statements of bank accounts
an employment contract or letter from an employer demonstrating you have employment beyond the end of your trip
evidence of attending school, or a letter from a school official demonstrating you will be a student there beyond the end of your trip
Getting visa help
A letter of invitation alone does not guarantee you will be issued a visa. If you have followed the above steps, and your visa application is rejected, let us know immediately. Immigration officials in Massachusetts will contact your embassy to try to overturn the rejection.
If you cannot afford the cost of visas or related fees, let us know. Financial assistance may be available.