United States citizens entering Turkey for tourism need to purchase a visa in order to enter. The tourist visa is good for up to 90 days and it requires an original, signed passport valid for 6 months beyond stay in Turkey, with at least one blank visa page available for a Turkey visa stamp. Amendment pages in the back of the passport are not suitable for Turkish visas, so make sure you have space.
Application outside Turkey:
One may apply for this visa in a Turkish consulate in the U.S. or abroad to get a visa sticker but this must be with significant time before the trip (several weeks) in order for it to be processed. Be sure and call the local consulate to determine exactly how much time you may need for the process.
Application at a Turkish border point of entry used to be the fastest and most efficient way to get the visa, but as of April 2014 this is no longer offered.
If you are traveling for tourism or commerce for up to 90 days within a 180 day period, you can apply for a visa online at www.evisa.gov.tr or at any Turkish Embassy or Consulate. The e-Visa replaces the sticker visa previously obtained at airports and other ports of entry. The cost for the multiple entry visa on this site is $20.
Various types of other visas are available at the consulate and cost more. They include single entry valid for a year $60, multiple entry for 5 years $200.
There is one exception to the visa requirement: U.S. citizens who are traveling to Turkey by cruise ship are allowed to enter Turkey without a visa for a maximum period of 72 hours, with permission given by the local security authorities at the port of entry.