Reality Check: Does EU membership make it harder to control immigration?
The claim: Home Secretary Theresa May says: "Free movement makes it harder to control immigration, but it doesn't make it impossible to control immigration.
Reality Check verdict: Free movement does make it harder to control immigration. But leaving the EU would not necessarily mean abandoning free movement, and the government has not managed to meet its immigration ambitions even for non-EU migrants, over whom it has greater control.
Freedom of movement means people holding the passports of European Union member states may go and live, work or study anywhere else in the EU.
This compares with migrants from outside the EU, for whom the UK currently has a points-based system that allows in people with skills needed in the economy as well as students and some temporary migrants.
The UK is not signed up to the Schengen agreement, so passports are still checked at the borders.
EU passport-holders are not automatically allowed in, but the bar is set quite high if EU citizens are to be rejected - just having a criminal record is not enough, they must pose a current threat. Last year, 2,165 EU passengers were turned away.
The latest figures from the Office for National Statistics suggest the UK's annual net migration - the number who have come minus the number who have left - from the EU stands at 172,000.
From outside the EU, it is 191,000.
The Conservative manifesto in 2015 said the party would "keep our ambition of delivering annual net migration in the tens of thousands, not the hundreds of thousands".
Clearly, even for just the non-EU migrants, over whom the government has greater control, the country is still well over that limit.
It should be said Norway and Switzerland also currently accept free movement, despite not being part of the EU, in order to gain access to the single market.
But Justice Secretary Michael Gove said last week the UK could be part of the European Free Trade Area, allowing it to trade with the EU without having to accept free movement.
Even so, it is possible, if the UK were to leave the EU, an eventual trade deal would mean it still having to allow favourable terms for EU migrants.