If you are paying by credit card in Spain and your home currency is not the Euro, watch out for the practice by some hotels, restaurants and shops of attempting to charge you in your home currency. The rate offered will not be preferential and may be treated as a cash advance by your bank, resulting in a daily interest charge, from the moment of payment. If offered, the phrase 'Quiero pagar en Euros, por favor' is a useful weapon.
Before you depart, do your best to choose a credit card provider who does not charge commission on overseas transactions and uses the current bank rate (not the less advantageous tourist rate).
Be aware that a credit card will not be accepted for the purchase of Euros at most banks, hotels and currency exchanges. You may use a debit card in a ATM machine, but not a credit card. Some shops and small restaurants/cafes do not accept credit cards.
NB: Most shops require formal identification when using your credit card; your credit card will not be accepted without it.
