A referendum asks you to vote yes or no to a question or proposal.
The people of Scotland will be asked the simple question: "Should Scotland be an independent country?"
Scotland has been part of the UK for more than 300 years, so the vote is a huge moment for the country.
For the first time, people aged 16 and 17 will be allowed to vote in the referendum. Usually in the UK the voting age is 18.
On Thursday 10 September, thousands of them gathered at a big debate in Glasgow, to ask some of the leaders some important questions.
All will become clear on 19 September, when it's expected that the result of the historic vote will be announced.
On 18 September, people across Scotland will face a massive choice - whether or not to become an independent country, separate from the United Kingdom.