Currency

Spain's currency is the Euro.

Exchange

The best thing is to change in offices of change, banks and other official places, to avoid possible frauds. In almost all the banks are a window of change that he/she will say canvi or currency estrangera, and the commission that get paid it is usually smaller than that of the offices of change.
In the main stations of trains and buses it usually has also offices of currency change with special schedule.

Credit cards

Without a doubt the most common use is that of the VISA cards, MasterCard and American Express.