Passport
You will need a valid passport. Americans do not need a visa to visit the EU.

Medicines
Take your prescription with you if you are carrying prescribed medicines. Do not exceed the quantities needed for your personal use during your trip, as large quantities of drugs can create suspicion.

Travel Insurance
You may want to take out travel insurance as only some EU countries pay the full cost of medical treatment. Illness or an accident abroad may mean extra travel, accommodation and repatriation costs, for which you may want to be insured.

Immunizations
There are in general no immunisation requirements when travelling in the EU.