First, check the current validity of your passport.
All travelers will need a passport valid for at least 90 days following your departure date from Greece. However, we strongly recommend traveling with 6 months validity on your passport at all times. VisaHQ can assist U.S. citizens with a full range of expedited passport services, including new passport application and passport renewal.
Second, make sure your passport has blank Visa pages.
Most destinations, including Greece, require that you have adequate un-used pages in your passport, allowing for any necessary stamps upon arrival and departure. We recommend that you have at least two free pages in Visas section of your passport before any international travel. U.S. citizens can get extra passport pages added to their passports as fast as within 24 hours.
Finally, confirm if transit visa is required for any connections.
Check with your airline in case you have connecting flights overseas as part of your journey to Greece. It may be the case that countries you pass through en route to your destination may require a separate transit visa. Please refer to visa requirements of the specific country.