working with a student visa?

Is this legal? I am non-EU (American) and I am doing a course in Barcelona on a student visa. During this time I have the oportunity do work in my field, but I am wondering, is this legal for me or the person employing me? I am in a situation where I could enter into some work, but I want to be prepared with the right information about legalities before I do so. If anyone has information, please let me know.

Thanks!
Luis