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.