Theresa May has officially stood down as leader of the Conservative Party on 7 June.
It's after her failure to get her deal for Brexit - how the UK will leave the European Union - through Parliament. Now, a new prime minister will take over once one has been chosen.
She said being the prime minister was a huge honour, but that it's time for someone else to take over.
Keep reading to find out how a new leader will be chosen.
Unlike in the US, for example, the public don't vote directly for the prime minister. That role is usually taken by the leader of the political party with the most Members of Parliament - in this case the Conservative Party.
Each party has its own rules for how it picks its leader. When that party is in government, the leader becomes the prime minister.
There will now need to be a leadership contest in the Conservative Party to decide who will replace Theresa May. She will stay as prime minister whilst that leadership contest takes place.
To have a full contest, there must be at least three people who want to take on the role.
Each would-be prime minister must have the support of at least two MPs to be considered a candidate.
They hold a series of speeches, or debates or events known as hustings where they set out their ideas and plans for what they would do if they were leading the country.
Conservative MPs in Parliament then vote on who they think are the best candidates.
The top two candidates will then go against each other in another vote, in which every member of the Conservative Party around the country - not just MPs - will take part.
The winner then becomes the leader, and in this case, the new prime minister.
Theresa May has become unpopular in her own party because of Brexit.
Despite working hard to find a way forward, she's been unable to get the support of MPs for her plan for how the UK would leave the club of countries called the European Union.
In March she even offered to quit if her Brexit deal was supported by Parliament, but it wasn't approved.