Greece is a party to the Schengen Agreement. Under the agreement the citizens of the countries outside the Schengen Agreement zone must meet some additional requirements.

If you are a U.S. citizen, here are the requirements:

For tourist travel: U.S. Passport must be valid 3 months beyond intended stay, no visa is required for stay up to three months (a maximum of three months cumulative stay during any six months period in the combined Schengen States), you must present tickets and documents for return or onward travel. No vaccination is required.

For business travel: U.S. Passport must be valid 3 months beyond intended stay, no visa is required for stay up to three months (a maximum of three months cumulative stay during any six months period in the combined Schengen States), you must present tickets and documents for return or onward travel. No vaccination is required.

For diplomatic travel a visa is required beside the above conditions. If you want to work or study in Greece you must contact the nearest Embassy or Consulate.