Nationals of the USA, Canada, most Western European countries, Australia and New Zealand do not need visas to visit Argentina. *** However, as of 20 December 2009, US citizens flying into Ezeiza International Airport must pay an entry fee of $US131. This fee is valid for ten years and allows for multiple visits to the country. For more details, see the US Embassy website or Lonely Planet's Thorn Tree travelers' forum. ***
In theory, upon arrival all non-visa visitors must obtain a free tourist card, good for 90 days and renewable for 90 more. In practice, immigration officials issue these only at major border crossings, such as airports and on the ferries and hydrofoils between Buenos Aires and Uruguay. Although you should not toss your card away, losing it is no major catastrophe; at most exit points, immigration officials will provide immediate replacement for free.
Dependent children traveling without both parents theoretically need a notarized document certifying that both parents agree to the child’s travel. Parents may also wish to bring a copy of the custody form; however, there’s a good chance they won’t be asked for either document.
Very short visits to neighboring countries usually do not require visas. Despite what a travel agency might say, you probably don’t need a Brazilian visa to cross from the Argentine town of Puerto Iguazú to Foz do Iguaçu and/or Ciudad del Este, Paraguay, if you return the same day, although you should bring your passport. The same is true at the Bolivian border town of Villazón, near La Quiaca. Officials at the Paraguayan crossing at Encarnación, near Posadas, have been known to extract cash ‘fees’ from crossers who don’t have a Paraguayan visa (British, Australian, Canadian and US nationals need them), assuming you stop or get stopped at the border.
For a 90-day extension on your tourist visa, visit the Dirección Nacional de Migraciones (immigration office;011-4317-0200; Antártida Argentina 1355; 7:30am-1:30pm Mon-Fri) in Buenos Aires. You must do so during the week that your tourist visa is scheduled to expire. The fee is AR$100.
Another option if you’re staying more than three months is to cross into Colonia or Montevideo (both in Uruguay) or into Chile for a day or two before your visa expires, then return with a new 90-day visa. However, this only works if you don’t need a visa to enter the other country (Australians need visas for Uruguay).