The Schengen Nations of Europe

As of March 2024, below is a current list of the 28 European Schengen Nations. They allow registrants to visit:

Austria
Belgium
Bulgaria (Mar 31, 2024: Borderless via Air and Sea only, not by Land or Train)
Czech republic
Denmark
Estonia
Finland
France
Germany
Greece
Hungary
Iceland
Italy
Latvia
Liechtenstein
Lithuania
Luxembourg
Malta
Netherlands
Norway
Poland
Portugal
Romania (Mar 31, 2024: Borderless via Air and Sea only, not by Land or Train)
Slovakia
Slovenia
Spain
Sweden
Switzerland

As an agreement, Schengen was signed among the five out of ten countries of the European Union members back then, on the 14th June 1985.  Under the Schengen agreement, travelling from one Schengen country to another is done without any passport and immigration controls or any other formalities previously required.