The Prime Minister David Cameron has said that if the Conservatives win the next election, voters will have their say on whether the UK should stay part of the European Union.
The EU is the group of 27 countries whose governments work together to make laws which affect all of our lives.
He said: "It is time for the British people to have their say.
"It is time to settle this question of Britain in Europe. I say to the British people: this will be your decision."
Mr Cameron says that before a vote is held, he wants to talk about how Britain's involvement in Europe could change.
Ed Miliband, the Labour leader, said that he doesn't think there should be a vote on Britain's membership.
He says it's a "huge gamble" that will cause years of uncertainty - and could harm Britain's economy.
Britain in Europe
The European Union or EU started life at the end of the 1950s as a club of just six countries - since then, others have joined over the years to make a total of 27 countries now.
Britain joined in 1973 and has been part of Europe for 40 years. From the beginning, politicians have argued about whether it's a good thing for this country, or not.
European laws affect lots of things in this country - from how many hours people spend at work, to how we produce food and catch fish to eat.
But leaders in other European countries like France say that Britain can't pick and choose its involvement in Europe.