Tipping your guides

I couldn't agree with Nick more! It is a different situation if you are taking one of the "free tours" where the guide is only paid on the basis of tips. If you have agreed upon a price, then that should be it.

I agree with Nick!
We don't need this American style of tipping in Europe

Thank you everyone for clarifying. As these are private tours, no tipping is the way to go!

If you liked your tour and your guide did an exceptional job, go ahead and tip. It doesn't have to be much, but 10€ would be gratefully received. Or buy them lunch. Many private guides may also working for someone else. Ask them.

Tipping is not just an "American" practice. What is American, is over-tipping.

Well, on tipping in Spain my advise to my guests is always not to feel "forced" to tip, unless the service has been very good and somebody has done a much better job than expected. It really depends on your "level of happiness". We do tip in Spain, based on how happy we are with the service: it may be just the spare change in a bar, or a few euros (10 or 15 if you are spending 100eur) or nothing. If you´re going to tip, it´s because you do feel it´s the right thing to do for someone that has offered an excellent service.

I totally agree with @MikelBasqueGuide