In Hartlepool, there will be a by-election - but what does that mean?
A by-election is an election that takes place if a seat in the House of Commons becomes vacant between one General Election and the next.
The Chief Whip of the political party whose MP held the vacant seat begins the process of a by-election.
This starts by 'moving the Writ', which is a motion requesting: ""that the Speaker do issue his Warrant to the Clerk of the Crown to make out a new Writ for the electing of a Member to serve in this present Parliament for the constituency of .... in the room of..."
If an MP dies, is recalled or becomes ineligible to sit during a Parliament, an election is held in the affected constituency to choose a replacement.
A by-election does not have to take place if an MP changes political party.
The conservatives are leading Labour by 17 points in the Hartlepool by-election.
The prime minister is set to add another blue brick to Labour's northern red wall with a victory there.
A poll for Good Morning Britain ahead of Thursday's by-election found the Conservatives have moved well ahead, in what has traditionally been a crucial Labour stronghold.
At the weekend we told how the shadow Foreign Secretary admitted "anything is possible" in the upcoming results.