There are 26 European countries that are party to the Schengen Agreement, which eliminates all internal border controls between them. Once you enter one Schengen country you may travel continuously for up to 90 days within the member countries. Within the Schengen area, you do not show your passport when crossing country borders.
If you are traveling for business or tourism:
Note: Travelers for business or tourism are permitted to stay in the Schengen area for 90 days within a six month period. Once the 90 day maximum is reached, leaving for a brief period and re-entering the area does not entitle a traveler to 90 more days within the Schengen states. The traveler would have to remain outside of the Schengen zone for 90 days before reentering without a visa. Immigration officers at the port of entry have the right to determine whether your planned activities are consistent with business or tourism. You should check with the Embassy or Consulate of the country to which you are traveling if you have questions about whether your proposed trip qualifies for visa-free travel. U.S. Embassies cannot intervene on behalf of U.S. citizens who are denied entry into a foreign country.
If you are traveling for reasons other than business or tourism (such as employment, study, internship, etc.):
Although European Union regulations require that non-EU visitors obtain a stamp in their passports upon initial entry to a Schengen country, many borders are not staffed with officers carrying out this function. If you want to be sure your entry is properly documented, you must ask for a stamp at an official point of entry. Without the stamp, you may be questioned and asked to prove how long you have been staying in Schengen countries when you leave.
The member countries of the Schengen agreement are: