Committees scrutinise the work of government departments
Parliament's select committees exist to investigate various aspects of public policy and society.
Their work involves calling witnesses with expertise and experience in the field of the inquiry and they will often produce reports.
Select committees play a vital role in the scrutiny of government and their investigations can trigger major reform.
The oral evidence sessions - around a horse-shoe shaped table - are at the heart of committee inquiries can lead to even the most senior government ministers squirming in their seat.
There is great competition for membership on the more important committees and it is up to the Committee of Selection to select their make up.
Non-departmental select committees date back to 1861, when the Public Accounts Committee was established.
Most non-departmental committees are concerned with the administration of the House of Commons, referred to as "committees of the House".
The present system of departmental select committees was set up in 1979 to shadow and report on the work of government departments.
The fourteen original departmental committees have increased in line with general principle that each department of state should have a committee shadowing it.
The size of committees varies but most have 11 members and reflect, roughly, the relative party sizes in the Commons.
Select committees can be made up of MPs, peers, or in the case of the joint committees a mixture of both.
By convention, no government ministers or front bench opposition members are committee members.
The Committee of Selection decides the membership of committees in the opening stages of a Parliament but they make their decisions after the party whips have suggested names.
Members are chosen for the whole Parliamentary term but the make up of a committee may change as members become ministers or frontbenchers.
It is the committee itself which chooses its chair. Some have chairs from opposition parties and there is a principle that committees should work in a non-party political manner.