International Drivers Permit in Spain
For those intending or considering driving in Spain, it is very important to know that Spain does require an international driver's permit for American drivers in addition to your US driver's license. The driver's permit is simply a translation and certification of your license, not a license itself. You must get this from a AAA office in the US before coming to Spain or else you are going to be in for a lot of hassle an delay trying to get one here and, essentially, you just cannot get one.
Can you drive without a permit? Yes. You can often get away with this. You only need one when the guardia civil pull you over, which is rare, or if you are involved in an accident. And if they do that, many people report that it is rare for them to ask for the permit. So even if that happens, you might get lucky, but they have ever right to demand one and if you do not have one you are going to have a rough time. It is not the same as driving without a license, but it is still going to be a pretty big pain.
Many people point out that when renting a car you do not need one and they think that this implies that you therefore do not need one to drive. This is not true and it is in no way the responsibility of a car rental company to verify that you have that kind of paperwork before renting you a car. That is between you and the guardia civil, it has nothing to do with the rental company.
But, it should be noted, some car rental firms do indeed require proof of the international driver's permit for Americans attempting to rent a car in Spain. Notably, in our experience, Gold Car requires it while Avis and Firefly do not. So your mileage may vary. Better safe than sorry, get it in the US before traveling and make your life that much easier. It is cheap and easy to get in the US before you leave.