European banks have switched over to a more secure credit card authorization process with a chip and PIN system.  US banks have not yet made this switch.  So while our country catches up with the technology, it is wise to bring multiple credit card with you when traveling to Europe.  The European credit card machines are still supposed to take both types of cards, but the people working at the places where you use your card may not understand that our cards still need to be run through the machine to read the magnetic strip.  They may need to try more than once to get the charge to go through.  We have not heard of major issues using ATM machines, but travelers checks can be a good back up.