The State Department’s travel.state.gov has a wealth of information for Americans abroad. For Country Specific Information please click on the following link:
Special Note: US citizens who are travelling to Greece or transiting through Greece on a red official or black diplomatic passport Must get a Schengen visa in their passport prior to departure. Greece enforces this visa requirement very strictly, and passengers without visas will be held at the airport police station and will be turned back to their point of departure. US citizens traveling on blue tourist passports do not need visas for stays of up to 90 days. If you have both a tourist and diplomatic/official passport, it is recommended that you travel with both.
Military and civilian members of the US Department of Defense, including dependents and contractors, are advised to refer to the DoD Foreign Clearance Guide https://www.fcg.pentagon.mil and the ODC country clearance website Here for official travel in Greece