People in the United Kingdom will vote on whether or not they should stay in or leave the European Union on 23 June 2016. The United Kingdom has been a member of the European Union for 40 years.
What is the European Union (EU)?
The European Union is a group of 28 countries in Europe whose governments work together.
It began after World War Two with the idea that the best chance of stopping another war was by countries working more closely together.
Each country has to pay money to be a member, which they mostly do through taxes. The EU uses the money to change the way people live and do business in Europe.
Certain rules to do with immigration have been relaxed too – so people from EU countries can move around more freely inside the Union.
Why do people want the UK to leave the EU?
There are many reasons, but a big one is that some people believe Britain is being held back by the EU, which they say forces too many rules on the UK.
They also want Britain to limit the number of people coming here to work from other European countries.
Why do people want the UK to stay in the EU?
They believe Britain gets a big boost from being a member of European Union – it makes buying and selling things to other EU countries easier.
They also believe Britain is stronger than on its own.
Prime Minister David Cameron described the referendum as one of the biggest decisions the country would face.