Do you tip in Portugal?

I’m such a bad traveler. I’ve been so busy that I didn’t have time to take a quick look at one of those travel guides that tell you about the basics of a country. So I’m here and have a bunch of questions.  First, I need to know about the local tipping customs. Are you supposed to tip?  When? how much?

We Americans need all the good PR we can get, so I’m happy to be generous. I just wish I knew what generous means.