Refugees and terrorism have put one of Europe's main achievements under enormous strain: the ability to travel through 26 countries without a passport.
EU law allows 400 million residents, plus many non-EU nationals, businessmen and women, and tourists, to travel freely across thousands of miles from the Russian border to the Atlantic coast of France or Portugal.
But the arrival this year of hundreds of thousands of refugees fleeing war in Syria, and Friday's attacks on Paris by suicide bombers who drove down the highway from the Belgian capital Brussels, have sparked calls to bring back Europe's internal borders.
They were removed by the Schengen agreement in 1985. Restoring them would mean showing passports again whenever a border is crossed. It would mean long lines, slowing down tourists and trucks.
It also could mean the end of the Schengen Visa, forcing tourists to apply for individual documents for each country.
At a time when Europe's economy needs all the help it can get, restoring border controls would raise the cost for business, deter tourists, and make it harder for people to live in one EU country and work in another.
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The Dutch truckers' association has estimated that adding an hour's wait at each border would cost its members an extra $684 million a year.
Trade would suffer too. In a paper published last year, Dane Davis of CRU Group and Thomas Gift of Duke University calculated that where two countries are members of Schengen, their total trade increases by 0.1% a year -- in the case of Italy and Spain that amounts to $50 million annually.
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But Schengen means much more than just driving across borders without slowing down. It's a cornerstone of modern Europe.
"Schengen is hugely important symbolically," said Vincenzo Scarpetta of think tank Open Europe. "It's the other big flagship project along with the euro."
Most people cite freedom of movement when asked what the EU means to them, and rate it as the EU's second biggest achievement after securing peace.
Border security will be a big issue at an emergency EU meeting on Friday. France says it will renew its calls for greater scrutiny of all travelers -- EU and non-EU nationals alike -- when they cross Europe's external border.
Some controls are already creeping back
The Schengen agreement does allow countries to reintroduce passport controls in certain circumstances, for a limited period of time.
Before the Paris attacks that killed 129 people, France had already suspended Schengen for the Paris climate summit to be attended by world leaders next month. Poland suspended the agreement for a major soccer tournament in 2012, and Iceland suspended it for a visit by the Hells Angels in 2009.
This year, Germany, Austria, Sweden, Hungary and other countries put temporary border controls in place to stem the influx of refugees who have streamed across the Mediterranean. Hungary has even built a wall on part of its border with Croatia, forcing refugees towards Austria.
Some countries have complained that migrants entered Greece -- which is inside the Schengen area -- overwhelmed the authorities there, and then were able to travel to other countries without checks.
The EU has said any border controls must only be temporary spot checks, not walls or resurrected border posts, and should not last more than 30 days.
Related: The list of suspensions of the Schengen Agreement
Schengen does not equal the EU
Not all EU members have signed the Schengen agreement. The United Kingdom and Ireland maintain permanent border control for visitors arriving from other EU countries.
Non-EU countries Iceland, Norway and Switzerland are in the passport-free travel zone, while new EU entrants Cyprus, Croatia, Bulgaria and Romania must join eventually.
European Commission President Jean-Claude Juncker has called for a Europe-wide border and coast guard system, instead of leaving it to individual countries to police the external frontier of the European Union. He says that would make it harder for people to enter the Schengen area illegally.
