Official documents:
We are not responsible for information regarding documentation necessary for your travel. Please consult with the appropriate consulate.

Following are general tips:
We recommend that you have passports for all passengers (including ALL children) and that they are valid for at least six months after your departure date. US citizens will be required passports for traveling to Europe, although not all countries may require visas for short visits (length of visit varies).

We suggest reading more about entry and visas requirements on Department of State’s Travel webpage.
http://travel.state.gov/visa/americans1.html