You've been hearing about Article 50 for a while, after Britain voted to leave the European Union last June.
It's the name of the legal process which Prime Minister Theresa May has now triggered.
It's never been used before by any EU nations so we asked some experts to explain the process.
"The prime minister will notify Donald Tusk in writing," 10 Downing Street explained to Newsbeat.
It's a "completely unprecedented and unregulated process," says Michael Dougan, professor of European law at the University of Liverpool.
"No doubt there are civil servants and diplomats in the UK and the EU who have developed some rough common understanding of what will come next."
About that letter from Theresa May...
Mrs May's letter was sent to Donald Tusk, the president of the European Council.
It was delivered at lunchtime on Wednesday by the British ambassador to the EU, Sir Tim Barrow.
Theresa May then announced to parliament to let the country know that Article 50 had officially been triggered.
"It is my fierce determination to get the right deal for every single person in this country," she said.
Now the UK has two years to exit the EU - which means untangling Britain from hundreds of laws passed over the last 40 years.
...and then a response from the EU
An EU official told Newsbeat that they did not know in advance what would be in the letter from Theresa May to Donald Tusk.
After its delivery, President of the European Council Donald Tusk said to Britain: "We already miss you. Thank you and goodbye."
Some of the main negotiating points for the next two years will be on big issues like immigration and trade.
The 27 other countries in the EU then have to agree on a response to Britain's outlines, and Donald Tusk will announce their framework for the negotiations "within 48 hours", according to the prime minister's spokesperson.
But the EU official says that the response "will not be too technical".
After all, we've still got two years - starting today - to get down to all the nitty gritty of the deal.