U.S. citizens traveling to the Schengen area for personal travel or business do not require a visa if you will be spending less than 90 days in a six-month period in the Schengen area countries. While a visa is not required for visits of less than three months, it is still important to check the expiration date on your passport before you travel. The countries in the Schengen area require your passport to be valid for at least three months beyond your planned departure date. The reason for the three-month validity requirement is that some of the countries in the Schengen area will assume that all travelers will stay the maximum three months allowed for visa-free visitors. This requirement may also exist for a flight layover while traveling from a Schengen airport to a non-Schengen destination.
The consequences of not having a passport valid for that length of time could include being refused boarding by your airline at the point of entry or when transferring planes. Additionally, there is the possibility of being denied entry upon arrival. For these reasons, the U.S. Department of State recommends that your passport have at least six months remaining when you travel abroad.