MPs are required to declare in the register of members' interests any extra income or gifts which they receive which could influence his or her actions, speeches or votes in Parliament.
There is a separate register for Lords' interests, which operates along similar lines.
When a parliamentarian takes part in a debate that may affect a business with which he or she has a financial connection, he or she must declare this interest to the House before speaking.
Following the furore over Tory MP Derek Conway's employment of his son, MPs are now also required to declare the names and job titles of any relatives they employ.
'Cash for Questions'
In 1994, after a national newspaper accused Conservative MPs of accepting money in return for asking questions in the House in what became known as the "Cash for Questions" affair, the Committee for Standards on Public Life was set up.
A compulsory register of members' interests had existed since 1974, but the committee, chaired by Lord Nolan, tightened the regulations on what information MPs are required to submit to the register.
The new register of members' interests, the first post-Nolan, was published on 7 May 1996.
It contained greater detail about the type of outside service which MPs provide and the amount of money they receive for this work.
However, a number of Conservatives refused to declare all of their earnings, instead providing only information about that portion of their work which they believe depends directly on their membership of the House.
MPs are only required to declare outside earnings related to the provision of services as an MP, rather than estimated on the basis of work "arising" out of their membership of the House (which is much more difficult to assess).
Lord Nolan's Committee also recommended on 8 July 1997 that there should be a criminal offence of "misuse of public office", applying to ministers, civil servants, police, magistrates, and judges who seek advantage from their public position.
Lord Nolan, in a press release coinciding with his Report on Standards in Local Government, stressed that the new offence would apply to all holders of public office (including MPs and councillors) and would encompass misconduct beyond only bribery and corruption.
Where MPs have declared fees received for parliamentary consultancy, they are required to deposit copies of these agreements and contracts in the Registrar's Office in the House.
The register is published annually and is available for public inspection.