Are you a non-Spanish person who drives here on an EU driving licence? As of last Saturday, 19 January 2013, there are new regulations for those who have been living in Spain for more than two years, and who hold a driving licence issued in an EU country.
You now need to have a medical examination every 10 years, until you're 65 years old, and then every five years. If you don't have the exam, you could find yourself facing a 200 euro fine.
The exam consists of a hand-to-eye coordination test, and a sight test. You can find a list of "Centros de Conocimiento de Conductores", where you take the medical test, here. If you pass, the Direccion General de Trafico will register you on their own system, and you will subject to the Spanish penalty points system, which covers offences such as speeding, using a mobile phone while you're driving, running red traffic lights, and drink-driving.
The good news is, you can still keep your EU licence, though you will need to provide a photo for Trafico's database; if you prefer to change to a Spanish licence, you need to go to your DGT.
The EU directive which has neccessitated these changes is 2006/126/CE; the new regulations will affect an estimated one million EU-nationality drivers resident in Spain.