The composition of the Government is entirely in the power of the Prime Minister.
When a Prime Minister rearranges his Ministers, dismissing some and bringing in new blood, this is known as a government reshuffle.
Reshuffles normally happen about once a year, although major ones tend not to take place in the two years preceding a general election. During a major government reshuffle, it is not uncommon for between a third and a half of the government to change jobs.
Reshuffles also follow a Ministerial resignation, allowing for the vacancy to be filled.  The filling of one Ministerial vacancy, can often lead to another Ministerial vacancy, as an existing Minister is moved to fill the newly found vacancy.  Thus when one Minister resigns, particularly at a senior level, this can lead to a number of changes lower down in the government.
A full government reshuffle is often hotly anticipated and is marked by extensive media speculation as to the winners and losers, based on rumour, off-the-record briefings and who arrives and leaves by which door in Downing Street.
Ministerial entrances and exits are confirmed via Number 10, with news about Secretaries of State normally coming first, and with details of Ministers of State and Junior Ministers then following later. The whole process may take several days and normally comes towards the end of the week.
Major reshuffles tend to take place in the summer, at the beginning of long Parliamentary recesses.
