In 1973, people dressed like this:

Slade ruled the charts

and Newsround was only 1 year old.

And that's when the UK joined what is now known as the European Union or EU.

It's a club of 28 countries from around Europe who work together.

This is its symbol. It's got its own parliament, its own anthem,

and its own money, the euro, used by most of its members, but not the UK.

Countries joined the EU because they think it will help bring Europe closer together.

But now British prime minister David Cameron has promised

to hold a vote on whether or not the UK should stay in or leave the EU.

Supporters of the EU think the UK is stronger as part of it.

That's because we buy and sell lots of things with European countries.

People who want to leave the EU say it forces too many rules on Britain.

They say leaving will give British people more control over issues like immigration.

It will be up to adults around the UK to decide what's best when they vote this summer.