Police and Judicial Co-operation
To counter the potentially aggravating effects of the abolition of border controls on undocumented immigration and cross-border crime, the Schengen acquis contains compensatory police and judicial measures. Chief among these is the Schengen Information System (SIS), a database operated by all EU and Schengen states and which by January 2010 contained in excess of 30 million entries. The Schengen Agreement also permits police officers from one participating state to follow suspects across borders both in hot pursuit and to continue observation operations, and for enhanced mutual assistance in criminal matters.
The Schengen Convention also contained measures intended to streamline extradition between participating countries however these have now been subsumed into the European Arrest Warrant system.
Read more about this topic: Schengen Area