President Bush's controversial trip to the UK is the first state visit by a US president. BBC News Online explains.
President and Mrs Bush will be staying at Buckingham Palace
What is the significance of a state visit?
A state visit is different from a normal visit in that it is organised by Buckingham Palace and involves a high level of pageantry.
This includes a ceremonial arrival, a full state banquet and a private lunch involving the Queen and her household and the president and his entourage.
Although the palace issues the invitation, it is done in consultation with Downing Street and the Foreign Office.
But surely he and other presidents have dined with the Queen in Britain before?
Many presidents - including Mr Bush - have dined with the Queen. Indeed, she has met every US leader elected during her reign - but their visits have not been formal state occasions.
Although President Bush will be the first president to stay at Buckingham Palace since Woodrow Wilson in 1918, several others have visited her at one or other of her residences.
So why now - given the depth of feeling against him and the Iraq war, isn't the timing unfortunate?
PREVIOUS PRESIDENTIAL VISITS
1961 - President John and Jackie Kennedy dine at Buckingham Palace
1969 - President Richard Nixon has lunch there
1982 - President Ronald and Nancy Reagan stay at Windsor Castle
1989 - President George and Barbara Bush visit
2000 - President Bill and Hillary Clinton have tea with the Queen
Why now, frankly, is the question many people are asking.
The timing is not very good for Tony Blair, who some observers think will suffer political damage from his close association with the president.
The huge demonstrations planned suggest it is not that good for President Bush, either.
But Foreign Secretary Jack Straw says he hopes the visit will illustrate the importance of Britain and Europe enjoying a close partnership with "one of the oldest and largest democracies in the world".
But isn't this really a political trip?
These trips are always party political. A Downing Street spokesman admitted the invitation was as much out of courtesy, given the countries' special relationship, as anything else.
PRESIDENT BUSH'S ITINERARY
Tuesday 18 November - Arrives and receives private welcome at Buckingham Palace
Wednesday 19 November - Meets Michael Howard and Charles Kennedy, gives speech on transatlantic alliance and meets UK families of 11 Sept victims before attending royal banquet with Queen
Thursday 20 November - Meets British soldiers who fought in Afghanistan and Iraq, holds meetings with Tony Blair at Downing Street and hosts dinner at US Ambassador's residence
Friday 21 November - Travels to Tony Blair's Sedgefield constituency to meet members of the public before returning to Washington
The two leaders are happy to support each other in times of trouble.
But according to Warren Hoge, of the New York Times, President Bush might not benefit from the trip all that much, either.
Pictures of huge numbers of people in Britain - America's closest ally - protesting on the streets will hardly enhance his re-election prospects.
Why have there been no previous US presidential state visits?
According to Buckingham Palace, it has merely been down to a desire in the past for lower-key ceremonial programmes.
The closest was Woodrow Wilson's visit in 1918, when he stayed with King George V just after Christmas.
But the bottom line, says the palace, is that a "state" visit is little more than a title agreed with the government and the White House.
"It is no more or less important than any other official visit."