Well, that depends on your satisfaction with the waiter and the meal:) it 's not compulsory to do it but normally if you have had a nice time to the bar/ restaurant you can leave something like less than 10%... For example if you spend something like 50€ eating tapas, 3-4€ it's enough, at least it's what we the spanish do....

There is really no obligation to tip in Spain. If you just get a coffee or breakfast or a beer, don't worry about tipping. If you have a nice big meal and you really thought the service was great, go ahead and tip a little. But just a little, nothing near the 20% we'd do here in the States. People working in restaurants in Spain are paid decent wages and are not counting on tips to pay their bills. You'll find the service is a lot more relaxed than in the U.S..... Enjoy your time in Spain!

One euro is ok. If the check is like 100€ and you are really pleased whith the service you could live 5€ (which is much more than most of us would leave no matter what service!). For bad service leave nothing. Waiters do not live on the tips, they have a "decent" wage