The ETIAS for US citizens allows Americans to visit any country within the Schengen Area.
The Schengen Area currently consists of 26 member states: 22 European Union (EU) countries and the 4 members of the European Free Trade Association (EFTA).
A further 4 EU countries are in the process of joining the Schengen Area (Bulgaria, Croatia, Cyprus, and Romania). ETIAS will allow US nationals to visit all of these countries too.
This means that ETIAS will grant Americans access to 26 out of the 27 EU member states, with only the Republic of Ireland maintaining a separate visa policy.
There are also 4 European microstates for which the ETIAS will be needed to visit visa-free from the United States. Although not technically part of Schengen, Andorra, Monaco, San Marino, and Vatican City can only be accessed by land from a neighboring Schengen country, for which ETIAS will be required.
US passport holders can use the ETIAS to visit the following countries: