The Schengen Agreement is aimed at removal of border controls but, following the refugee crisis of 2015 and the attacks in Paris on 13th November 2015, this agreement is coming under severe pressure. There are 26 countries in Schengen. 22 are EU members and four non-EU. Those four are: Iceland and Norway (since 2001), Switzerland (since 2008) and Liechtenstein (since 2011).
The UK and Republic of Ireland opted out of Schengen.
The UK wishes to maintain its own borders, and Dublin prefers to preserve its free movement arrangement with the UK - called the Common Travel Area - rather than join Schengen. Nevertheless, the UK and Ireland began taking part in some aspects of the Schengen agreement, such as the Schengen Information System (SIS), from 2000 and 2002 respectively.
- For a good explanation of Schengen see - European Commission - Schengen Area and BBC 14th September 2015 - Schengen: Controversial EU free movement deal explained
- Europa - Schengen Area and Cooperation
At the time of writing, changes to the Schengen Agreement seem likely in response to the terrorist threat - Telegraph 16th November - Paris attacks: France to call for effective suspension of Schengen open borders.
Built into the Schengen system is the right for a State to reintroduce border controls on a temporary basis in the event that a serious threat to public policy or internal security has been established. The reintroduction of border control at the internal borders must remain an exception and must respect the principle of proportionality. The scope and duration of such a temporary reintroduction of border control at the internal borders is limited in time and should be restricted to the bare minimum needed to respond to the threat in question. Reintroducing border control at the internal border should only ever be used as a measure of last resort. The reintroduction of border control is a prerogative of the Member States. The Commission may issue an opinion with regard to the necessity of the measure and its proportionality but cannot veto such a decision if it is taken by a Member State.