At a general election, all MPs stand for re-election and every constituency across the country chooses between available candidates. General elections generally happen every four to five years.
If an MP dies or retires, an election is held in that constituency alone to find a new MP for that area.
Most MPs are members of one of the three main political parties in the UK - Labour, Conservative and Liberal Democrat. Other MPs represent smaller parties or are independent of a political party.
To become an MP representing a main political group, a candidate must be authorised to do so by the parties nominating officer. They must then win the most votes in the constituency.
UK-wide representation and devolved Parliaments and Assemblies
The UK Parliament has MPs from areas across England, Scotland, Wales and Northern Ireland. In addition, there is a Parliament in Scotland, a National Assembly in Wales and a National Assembly in Northern Ireland.
Separate elections are held for these devolved political bodies (which have been granted powers on a regional level that the UK Parliament was responsible for) - candidates who win seats in these elections do not become MPs in the UK Parliament.