The Turkish Government imposes an 18% value-added-tax (KDV in Turkish) 

on virtually all goods and services.  The KDV should already be included in the price of

most items, including food.  You will often see the term "KDV dahil" to denote that KDV

is included.  KDV is usually included in the price of restaurant food as well and should

not be added to the price in that case.

It is customary to leave approximately 10% of the bill as a tip for meals

ordered in restaurants.  If you pay for the meal with a credit card you should leave the

tip in cash.  In fact, you will find that there is not even a place on the credit card slip to

add a tip.

Note that in some restaurants frequented by tourists you may find that a 10%

service charge is already in the total (it usually will say "Servis").  If you are in doubt as

to what the additional charges are don't hesitate to ask.

As noted above, taxi drivers do not expect tips and should not be tipped.  The

fare should be rounded up or down as necessary.