The legal currency in Argentina is the "Argentine Peso" (there is also the Mexican peso, and accross the River Plate the Uruguayan peso). The notes and coins have this appearence:

              Pesos notes and coins


DECEMBER 2015 UPDATE: the new government has lifted currency exchange restrictions and now dollars can be freely exchanged for pesos.

This is a new development and the exchange rate for U.S. dollars in February 2016 was about 15 pesos per U.S. dollar. Tourists should be able to pay with foreign credit cards, withdraw pesos from ATM's and exchange currency at banks and bureaus at a realistic, market based exchange rate.

Try to get some pesos as soon as you get to Argentina. Buses do not accept foreign currency. Taxis and shops might accept U.S. dollars, and they should give you a realistic exchange rate, but that can involve some negotiating.

Banks & Currency exchange

Currency can be exchanged in most banks (open from 10 am to 3 pm) and exchange companies "casas de cambio" operating in the same time-range. Be careful exchanging your money with people in the street, operating mainly in the downtown area (called "arbolitos" -little trees- by locals, since they are right beside the road and are full of "green" leaves) even though they might quote you a higher "informal"  exchange rate. If you do use them, try to exchange a small even number such as $100 to make  calculations easier (some are known to use "fixed" calculators in order to pay you less money).

If you arrive to Buenos Aires by plane, there is a branch of the Banco de La Nación (state owned bank) located in the arrivals area of the airport which will exchange your currency at the market rate.

You can always exchange dollars in several Banco de La Nación branches or at exchange bureaus across town. Another alternative is to get local money from your account using the ATMs spread all over the city, ATMs are found not only in banks but also in Shopping Centers (such as Alto Palermo, Paseo Alcorta, Unicenter, etc.) and many large supermarkets (Jumbo, Disco, etc.).  The largest network of ATMs is Banelco. In the web page you can find an ATM locator. The web page is in Spanish, unfortunately. Beware of the credit card rates.

ATM's - ATM's are numerous but travellers should be aware that there is a charge for withdrawing from a foreign credit/debit card. This is usually between AR$80-85 per transaction. Therefore the advice is withdraw the largest amount at each transaction. At some banks this can be as low as AR$700 so avoid these banks and go to where withdrawals of AR$ 2000 are possible.