What is the appropriate percentage to give someone for tip at a restaurant?

The American system for wait staff is for the restaurant to underpay them and for the rest of their pay to come from tips. The first 15% is basically their pay. If they do an adequate job they deserve 15%. If they do a good job they should get more. If they don’t do an adequate job than they should get less than 15%. I’ve only stiffed one waiter in the last 20 years; this was a guy what was more interested in hanging out with his friends at another table than getting my order, refilling my coffee, etc.; I actually gave up on him and went behind the counter to refill my own coffee. The restaurants I go to have good wait staffs so I usually tip 20%. 20% is also easier to calculate in your head which is handy. Just double your bill than move the decimal point one place to the left. I usually add another dollar or so if I end up sending my meal back or if the wait person comps something for me. I also tend to tip a higher percentage at inexpensive restaurants since I don’t think it is fair for wait staff to make less money just because the food is reasonably priced.

It depends on the country. In my country 10% is considered an adequate amount. Anything above that means good service.