General Requirements:
US citizens require a passport valid for at least three months after the end of their stay. No visa is required for a stay of up to three months if coming as a tourist.
Although European Union regulations require that non-EU visitors obtain a stamp in their passport upon initial entry to a Schengen country, many borders are not staffed with officers carrying out this function. If an American citizen wishes to ensure that his or her entry is properly documented, it may be necessary to request a stamp at an official point of entry. Under local law, travelers without a stamp in their passport may be questioned and asked to document the length of their stay in Schengen countries at the time of departure or at any other point during their visit, and could face possible fines or other repercussions if unable to do so.
If you plan to stay longer than the expiration date of your Schengen or national visa while in Greece as a tourist, student or temporary business consultant, it is wise to apply for a visa extension at the Alien’s Bureau Office or police station nearest to your legal or temporary residence here. The location varies according to municipality, and you will be sent away if you go to the wrong one.
Visa extensions are normally granted under special extenuating circumstances, such as being in the hospital, having a serious car accident or a relative passing away. Visa extensions are not granted because of ignorance regarding visas/permits, careless planning, unexpectedly falling in love, taking illegal work and just because you want to travel more.
General Consulates of Greece: