No country in the world is self-sufficient, and so everyone tries to commerce at the market goods and services that are necessary for the working of each economy. Those exchanges of elaborated products, raw materials or goods and services are transactions that are done in foreign currencies. Transactions in foreign currencies must be done using the exchange rate that used at the bank market at the moment that those operations are done. Transactions in foreign currencies are included in the perfect market as the supply and demand law works to determine the price or “exchange rate” for all the currencies around the world, so the demand is greater than the supply, the price raises, and vice versa, if the supply exceeds the demand, the price falls. One of the principal problems that transactions in foreign currencies carry out is that they must be done in a common currency among the involved countries. As in the definition is included the idea of a national currency, while doing a purchase or sale of goods we should determine the exchange rate of one currency to the other. Most commercial operations must be done with transactions in foreign currency and the major part is done in dollars so many countries are interested only for the relation between the value of the domestic currency and the value of the dollar. Today in Latinamerica there is a project between the countries members of the MERCOSUR (Common Market of the South), to eliminate the transactions in foreign currency and carry them out with a regional currency, everything is still being discussed but it is an initiative that would improve the commercial relations amoung the Latinamerican countries.