The Schengen Agreement abolished the EU’s internal borders, enabling passport-free movement across most of the bloc.
But the 13 November attacks by Islamic State (IS) jihadists in Paris, which killed 130 people, prompted an urgent rethink.
There was alarm that killers had so easily slipped into Paris from Belgium, and that some had entered the EU with crowds of migrants via Greece.
And in 2015 the influx of more than a million migrants – many of them Syrian refugees – greatly increased the pressure on Schengen.
One after another, EU states reimposed temporary border controls.
In December the European Commission proposed a major amendment to Schengen, expected to become law soon.
Most non-EU travellers have their details checked against police databases at the EU’s external borders. The main change is that the rule will apply to EU citizens as well, who until now had been exempt.
Non-EU nationals who have a Schengen visa generally do not have ID checks once they are travelling inside the zone. But since the Paris atrocity those checks have become more common.