Stats and facts
The free movement of people is one of the four founding principles of the European Union, and arguably the one most closely associated with EU membership.
It gives all citizens of EU countries the right to travel, live and work wherever they wish within the EU, and has contributed to frequent travel between member states and the decision by many Britons to buy homes or second properties within the EU.
In total, it is estimated there are 1.3 million Britons currently living in other EU countries , mainly in Spain, Ireland, France and Germany.
Europe is also the most popular holiday destination for Brits, with some 47 million visits in 2014.
As well as short trips, many Britons own holiday homes in the European Union with 200,000 estimated to have a second property in France alone.
What does the EU do?
All EU citizens have the right to travel, live and work wherever they wish within the EU, with guaranteed access to healthcare and public services.
In all EU member states, airline passengers are eligible for financial compensation if a flight departing from an EU airport or arriving in the eurozone with an EU carrier is delayed for a reason within the airline's control.
The European Health Insurance Card gives EU members access to state health services on the same terms as locals whichever country they are in. The card can be used within the wider European Economic Area (EEA), which includes the 28 EU countries as well as Iceland, Liechtenstein and Norway.
EU members studying at universities in other EU countries are charged the same fees as locals.
…and what doesn't it do?
There are no EU-wide property rights or taxation rules, these are determined country by country.
Each EU country also has its own rules for awarding unemployment and other benefits.
UK opt-outs of EU rules?
The UK is not part of the Schengen agreement that lets people cross internal borders without showing their passport. Nor has it adopted the euro.
The argument for leaving the EU
Given the importance of UK tourism for the European Union it is likely the UK would quickly renegotiate similar privileges to the ones British tourists and workers currently have, and expats who are already living in the EU are expected to keep that right. Leaving gives more power to decide who can live in the UK, as explored in our UK-EU immigration explainer.
The argument for staying in the EU
There is no guarantee that Britons' current rights to live, work, travel, own property and access public services such as medical treatment in the EU would be maintained, and their continuation would depend on new agreements the UK negotiated with those nations.