The short answer is, they can't.
In 1624 attending parliament was seen by some as a chore, a bit like jury duty.
So the Commons passed a resolution banning MPs from resigning their seats.
It states simply "that a man, after he is duly chosen, cannot relinquish".
To this day death, expulsion or disqualification are the only ways out.
An elegant solution is used to get round the problem, involving an automatic disqualification, the chancellor of the exchequer and titular stewardship over a portion of Buckingham.
Under the Act of Settlement, any Member of Parliament accepting an office of profit under the Crown must give up his or her seat.
So an MP who wants to resign from the Commons applies to the chancellor for one of two offices that exist just for this purpose: Crown Steward and Bailiff of the three Chiltern Hundreds of Stoke, Desborough and Burnham, and Steward of the Manor of Northstead.
They were retained as nominal offices of profit solely to meet the requirements of the House of Commons Disqualification Act 1975.
The former MP retains the office until the chancellor appoints another applicant or until the holder applies for release from it.
The offices are normally rotated, so Iris Robinson, the DUP MP for Strangford, who is to stand down next week, is likely to be appointed the Chiltern Hundreds.
Ulster unionists, inlcuding some from the party she was expelled from earlier today, staged a mass resignation over the Anglo Irish Agreement almost 25 years ago.
Fifteen of them were individually appointed to the stewardships for a few hours on December 17 1985, then stood for re-election on an anti-agreement platform.
The last MP to resign was former Speaker Michael Martin on June 22 last year. He is the current Steward of the Manor of Northstead.
Tony Blair, Boris Johnson, David Marshall and Ian Gibson, the current Steward of the Chiltern Hundreds, have all left the Commons in this parliament, along with David Davis, who resigned over detention without charge and was immediately re-elected for Haltemprice and Howden.
Other famous names to have taken the Chiltern Hundreds include Betty Boothroyd, Neil Kinnock, Brian Walden, Roy Jenkins, John Stonehouse, John Profumo, Sir Stafford Cripps and Austen Chamberlain.
Former Stewards of the Manor of Northstead include Peter Mandelson, Piers Merchant, Bryan Gould, Leon Brittain, Robert Kilroy-Silk, Matthew Parris, Selwyn Lloyd, Hartley Shawcross and Anthony Eden.
"Upon receipt of a Member's application for the Chiltern Hundreds, a warrant of appointment is signed (in the presence of a witness) by the chancellor of the exchequer," according to the parliament website.
"Since 1850, these have been registered and retained in the Treasury.
"On the day the warrant is signed a letter is sent to the Member, omitting the letters MP after his name, to inform him that he has been appointed to the office.
"Letters of notification are also sent at the same time to the offices of the Speaker and the government and opposition whips.
"As soon as practical, the appointment is noted in the London Gazette.
"It is also the practice for the Treasury to issue a brief press notice.
"The disqualification of a Member because of his new office is recorded in Votes and Proceedings although there are no proceedings in the house and it is not recorded in the Official Report.
"If a Stewardship is granted during a recess, the new writ for a by-election cannot be issued until the House meets again.
"If it is granted during the session, the party’s whip is free to move for a new writ immediately after the chancellor of the exchequer has signed the warrant of appointment."
