Britain will decide if it wants to remain a member of the European Union or leave it when a referendum is held on 23 June. It is more than 40 years since the last one on Europe. In 1975, Britons were asked whether or not they wanted to remain in what was then called the Common Market — and 67% said yes.
Ahead of the landmark Brexit referendum in June, here are 10 quick facts about the EU and the UK's membership of it.
1. There are 28 member states so far and a further seven described as "on the road to membership", including Turkey and Serbia.
2. The EU has 65 institutions, bodies and agencies.
3. Within the EU, there are 19 members of the eurozone currency area.
4. There are 751 members of the European Parliament (MEPs).
5. It is the world's largest economy with a nominal GDP of €18.5bn (£14.6bn, $20.2bn) for all 28 economies combined, says the International Monetary Fund.
6. There are around 507 million citizens living in the EU, about 7% of the world's population.
7. Just over three million people in Britain were born in other EU countries, or 5% of the whole population.
8. In 2015, the UK paid a net £8.5bn of contributions to the EU, making it the third biggest contributor behind France in second place and Germany in first.
9. The EU budget in 2016 is €144bn (£114bn). By comparison, public spending by the UK government alone in 2015-16 is expected to be £742bn – or 6.5 times the EU's entire budget.
10. There are 55,000 civil servants employed by the EU across its various institutions.