Citizens of European Union countries do not require a visa to enter Greece.  They only require a valid passport or an ID card (for Schengen countries)

Citizens of Andora, Argentina, Australia, Brazil, Brunei Darussalam, Canada, Chile, Costa Rica, The Holy See (Vatican), Croatia, El Salvador, Guatemala, Honduras, Israel, Japan, Malaysia, Mexico, Monaco, New Zealand, Nicaragua, Panama, Paraguay, San Marino, Singapore, South Korea, Switzerland, USA, Uruguay, and Venezuela, entering Greece for a period of less than 3 months require only a valid passport.  They do not require a visa.  For visits exceeding 3 months, authorization must be obtained in person by applying  to the Aliens Bureau at least 20 days prior to the 3-month expiration date.

Citizens of countries other than the above, should contact the nearest Greek Consulate office or Greek Embassy with regard to visa requirements. To locate the Greek Consulate office or Greek Embassy nearest to you, click here

Invitation Letter

If you require an Invitation Letter to apply for a visa to enter Greece, e-mail the following information to iau257-at-cc-dot-uoi-dot-gr

1. First and last name, exactly as written in the passport
2. Affiliation (institution name, address, phone, fax, e-mail)
3. Date of birth
4. Passport number
5. Country where the passport was issued 
6. Date passport was issued
7. Date of expiry of the passport 
8. Dates of your stay in Greece (e.g. September 12, 2008 - September 20, 2008)