MPs will make known that they want to ask questions and will be called by the Speaker of the House of Commons. The Leader of the Opposition has a guaranteed right to be heard, and has a greater allocation of questions (six rather than two) compared to the backbench MP. It is also customary to call on the leader of the Liberal Democrats at every session.
If the Prime Minister is away on official business then a substitute, usually the Deputy Prime Minister, will answer questions. It is customary that the Leader of the Opposition also sends a substitute.
Since the televising of Parliament "PMQs" have formed an important part of British political culture. Because of the natural drama of this confrontation it is the most well known piece of Parliamentary business. Tickets to the public gallery for Prime Minister's Questions are the most sought after Parliamentary tickets.
One of Tony Blair's first acts as Prime Minister was to replace the two 15 minute sessions, held on a Tuesday and Thursday, with a single 30 minute session on a Wednesday, a move for which he was criticised. He also increased the opposition leader's allocation of questions.