By the Schengen Agreement signed on 14 June 1985, Belgium, Germany, France, Luxembourg and the Netherlands agreed to gradually remove controls at their internal borders and to introduce freedom of movement for all nationals of the signatory countries, other EU countries and some non-EU countries.
The Schengen Convention supplements the agreement and lays down the arrangements and safeguards for establishing an area without internal border control. It was signed by the same five countries on 19 June 1990, and entered into force in 1995. The agreement and the convention, as well as the related agreements and rules, together form the ‘Schengen acquis’, which was integrated in the framework of the EU in 1999 and has become EU legislation.
Twenty-two out of 28 EU countries form the Schengen area. Bulgaria, Croatia, Cyprus and Romania will join the area in time. Ireland and the United Kingdom have opt-outs and maintain their border controls. Four further countries, Iceland, Liechtenstein, Norway and Switzerland, also participate in the Schengen area.
EU accession candidate countries must accept the whole of the Schengen acquis at the time of their accession. However, border control at internal borders is only lifted (by unanimous Council decision) after an evaluation by Commission and EU country experts has been carried out to ensure that all accompanying measures allowing for the lifting of internal border control are in place.
Source: EUR Lex Glossary