Question: Teaching English in Spain without a Visa
Answer: The whole of Spain is learning English and many schools are crying out for teachers. It should therefore be no problems getting work without a visa. However, you will not be covered by social security (which means hospital bills could be expensive) and you will usually earn a little less than someone with a work visa.
I have known a number of Americans teaching English in Spain and none have ever had any problems. However, in theory, if you get caught, you may be deported.
A good forum thread on this subject can be found at Notes from Spain. For a differing point of view, see this Warning for non-EU teachers in Spain.
If you are an EU citizen, you don't need a work visa.

