Passports and Visas
All U.S. citizens, even infants, need a valid passport and a visa to enter Turkey for stays of up to 90 days. Visas must be obtained prior to arrival by filling out an application via www.evisa.gov.tr. The cost is $20.
Even though visas are multiple entry and usually valid for 90 days out of a 180-day period, they cannot be issued for periods longer than the validity of the passport you present. If your passport has less than a month to run, or does not have enough blank space for entry and exit stamps, you may not be given a visa at all. Check the validity of your passport before applying for the visa. Turkish officials may impose stiff fines for an overstay on your visa.
If your trip includes a stopover to the Greek side of Cyprus before you come to Turkey, make sure you don't get your passport stamped. Instead, ask for a slip of paper indicating your legal entry to Cyprus. Otherwise, you may encounter difficulties getting through passport control in Turkey. This is not an issue if you're coming from Greece proper, however.