Border crossing between these countries is fairly relaxed - authorities assume most people are on a day trip across the border. US passport holders require a visa US$130 to visit the Brazilian side of the falls which is NOT issued at the border. European Union passport holders do not normally need a visa to enter Brazil for tourism. Better check before setting off as the Brazilian side of the falls is "a must".
Beware that there may be lines on either side of the border, depending on the time of the day and holidays.
Also check regarding immunization requirements. Australian residents, for example, require yellow fever shots if returning to Australia within 6 days following a visit to Brazil. Brazil also requires yellow fever immunization prior to entering if your passport shows that you have visited some specific South American countries Guyana, for example.
