Do US citizens need a visa for Turkey?
NOTE: Due to changing circumstances related to US citizens this answer is NOT currently correct. Turkey is no longer issuing e-visas to US citizens, and visa-on-arrivals are only available to US citizens under very restricted circumstances (non-US residents, and some people related to Turkish citizens).
Exact circumstances are changing every few weeks, so I would suggest researching the exact situation before visiting Turkey as any answers on Travel.SE may be outdated quickly
Yes you need a visa to enter Turkey, according to Turkish Ministry Of Foreign Affairs
United States of America: Ordinary and official passport holders are required to have visa to enter Turkey. They can obtain three month-multiple entry visas from Turkish diplomatic representations abroad and also via the website www.evisa.gov.tr.
Please note, no more visa stickers on border crossings, you have to obtain the e-visa in advance:
As of April 17, 2013, electronic visa (e-Visa) replaces “sticker visa” which was issued at the border crossings. Applicants just need to log on to www.evisa.gov.tr , provide the requested information, (after the application is approved) make online payment and download their e-Visa.
The e-visa is easy to obtain and takes 24 hours to be issued, as mentioned in the FAQ page:
Your e-Visa will be e-mailed to you within 24 hours after completion of your application.
Note: The e-Visa's introduced in 2013 are now becoming mandatory. Sticker visas will not be available at border crossings beginning April 10th, 2014. If you needed a visa before, you must now obtain it in advance. This includes US citizens. There are even more restrictions on arrival for many other countries.