Since 1999, 22 of the EU's member states — Austria, Belgium, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Poland, Portugal, Slovakia, Slovenia, Spain, and Sweden — have chosen to get rid of their own internal border controls.
Instead, they've formed the Schengen Area, named after the town in Luxembourg where the agreement was signed. For travel and immigration purposes, the area essentially functions as one big country, with common rules for visas, asylum requests and border checks. There's an exception built in the rules that allows a country to reimpose its own controls for a short time when there's a major security threat or some international event, such as a sports championship or a big conference of world leaders [source: Archick].
Despite the freedom of movement that the Schengen Area allows, it doesn't result in as much migration as you might expect. Politico's European edition reported in 2018 that only 11 percent of people in the EU are living outside the country of their birth, compared to 13 percent in the U.S. And many of those immigrants actually come from other EU countries [source: Cokelaere].
But the EU has faced growing internal tensions over what to do about refugees from the Middle East and Africa, nearly 2 million of whom have arrived since 2014. Italy and Greece have taken most of the migrants, in large part due to their geographic location. Some countries want refugees to be distributed more evenly among all nations. Other countries don't want to accept any immigrants and have pushed for more restrictive EU immigration policies [source: Henley]. The issue is threatening the "freedom of movement" agreement within the EU and was one issue behind Brexit.