Currently a visa is not required for US passport holders who are traveling in Europe for less than 90 days.
Recently, there have been talks about a change to this policy. No changes to the existing policy have been announced.
CISabroad is monitoring communication about changes to policies and procedures for US passport holders who are traveling in Europe. It is uncertain whether or not any changes will occur.
If any changes are announced CISabroad will communicate with applicants and with program participants as soon as possible. We are confident that any change to existing policies would give travelers ample time to take any new steps necessary to enter Europe. In the event of a change we will work with all CISabroad participants to make things as clear and easy as possible.
US tourism in Europe is a major income generator, so many European nations are advocating for no changes to be made.
More information can be found in this article from Foreign Policy Magazine.