There´s no need of carrying money in cash; most of the places will accept Visa or Mastercard Credit Cards.
The Brazilian currency unit is the 'Real' [pl. Reais], which is not very strong, but is remaining considerably stable during the last few yers. Using dollars is not permited in Brazil, so you will always have to change your money.
Although carrying some money in cash for small expenses is a good idea, there´s no need to taking large amounts with you one credit cards are largely accepted, from pubs to shows. The unique important exception are the taxis. Visa and Mastercard Credit and Debit Cards are accepted in more places than Dinners or American Express. So if you´re carrying one of this two, don't worry, you will never run out of money.
