An agreement between EU member states (and some non-EU members like Swizterland and Norway) to remove border controls.
It was originally signed in 1985 in the small eponymous village in Luxembourg where the borders of France, Germany and Luxembourg meet.
People can travel between countries in the so-called "Schengen area" without the need to show their passport at border crossings.
Instead of previous customs and security checks, most internal borders within the EU are largely unmarked.
The UK and Ireland have secured an opt-out from the agreement.
The Schengen Agreement was formally incorporated into EU law under the Treaty of Amsterdam.