Parliamentary elections in individual constituencies are called when an MP dies or resigns during a Parliament.
Other official causes for a seat to fall vacant - triggering a by-election - include the incumbent MP's bankruptcy, elevation to the peerage, or lunacy.
A legal document known as a writ must be issued by the monarch to authorise the holding of a by-election.
The Speaker issues the writ in the Commons Chamber on the application of a whip from the party which held the now-vacant seat. The writ is normally moved at the start of business at 1430 GMT.
By-elections can be an important test of the political parties' popularity.
In by-elections held between May 1993 and February 1997, as many as eight seats were lost by the outgoing Conservatives to Labour and Liberal Democrat candidates.
The 1997 to 2001 Parliamentary term was very unusual because the government did not lose any seats in by-elections.