Visa information is given on the ICM web site. In short, visas are required for citizens of the USA, Canada, India, and most Asian and African countries, while visas are not required for most European and Central/South American citizens (wikipedia list of exempt countries). The Brazilian visa system operates under a strict reciprocity policy, meaning that citizens from countries which require visas from Brazilians will have to obtain visas to attend the conference. Fortunately, a tourist visa normally suffices to attend a scientific conference in Brazil (stated on the San Francisco consulate page under Visa Types).
The initial visa application form (you may have to explicitly accept the SSL certificate) is on the web, but ultimately some individual (you, a friend, proxy, broker, …) must present the application in person at a Brazilian consulate (not by mail). Map of US consulates. Non-US participants should get in touch with the nearest Brazilian consulate (click on the map) to find where to apply.
If you need an invitation letter, please email us.
Sponsored by the TeX Users Group, DANTE e.V., ICM'18, and individual donors. Thanks to all. New sponsors are greatly appreciated, as is mentioning the conference to colleagues and in any other contexts.
Contacting the organizers: