Schengen Agreement, signed in June 1985 in Schengen, Luxembourg is an agreement between participating European Union countries. The agreement allows people in European Union countries to live and work anywhere within EU. They can cross borders without a passport. The one requirement is a government issued ID card.
As US citizens, we can travel to EU countries with no visa, but it is limited to 90 days within a 6 months period.
It is an interesting problem as we plan our next overseas trip. J.J. and I are considering trips to Europe in 2018.
Aloha -- Cathi