schengen area

Twenty-two of the twenty-eight European Union (EU) member states participate in the Schengen Area.
Of the six EU members that do not form part of the Schengen Area,
four – Bulgaria, Croatia, Cyprus and Romania – are legally obliged and wish to join the area,
while the other two – Ireland and the United Kingdom – maintain opt-outs.
All four European Free Trade Association (EFTA) member states - Iceland, Liechtenstein, Norway, and Switzerland - have signed the Schengen Agreement, even though they are outside the EU.
In addition three European microstates – Monaco, San Marino, and the Vatican – can be considered as de facto within the Schengen Area as they do not have border controls with the Schengen countries that surround them;
but they have not officially signed documents that make them part of Schengen.