By Andrew Benson
BBC Sport analyses who has gained and lost from Juan Pablo Montoya's switch to McLaren in 2005.
Juan Pablo Montoya
On the face of it, there seems no convincing reason to swap Williams for McLaren.
The teams were evenly matched in 2003, and if either had an edge in performance it was Williams, notwithstanding the fact that McLaren's Kimi Raikkonen beat Montoya to second place in the championship behind Michael Schumacher of Ferrari.
On top of that, McLaren suffered the fiasco of their MP4-18, the "wonder car" that never raced because it was not reliable enough.
Montoya fell out with Williams over team tactics and money
However, Montoya knows as well as anyone the state of the Williams team and will be well aware that their BMW engine is considerably more powerful than McLaren's Mercedes unit.
He might draw from that the conclusion that McLaren's car is markedly superior to the Williams and therefore believe his new team is better placed for the future.
That conclusion, though, rides on Mercedes' ability to build a better engine than it has managed since it dropped off the F1 pace in 2000.
Still, Montoya will win from his decision in at least one way.
His relationship with Williams had weakened through 2003 after a row at the French Grand Prix and he grew to believe the team favoured team-mate Ralf Schumacher.
This belief may have been exacerbated by his growing frustration at the discrepancy between his salary and that of Schumacher.
The German earned £7.5m to Montoya's £4m in 2003, and the Colombian has upped that with his move to McLaren to £5.6m, undoubtedly with a promise of much more to come.
Verdict: Jury's out
McLaren boss Ron Dennis has put his team in a strong position
McLaren have put themselves in a very strong position as they seek to end Ferrari's domination of Formula One.
With Michael Schumacher probably no more than three years from the end of his career, McLaren have under contract two of the three men most likely to assume his crown - Montoya and Kimi Raikkonen.
Only Fernando Alonso remains out of reach, and the Spaniard is locked in at Renault for the foreseeable future.
Even though McLaren did not reveal the identity of Montoya's partner in 2005, it is no secret that he will team up Raikkonen, making the strongest overall driver pairing in F1.
All McLaren have to do now is come up with a competitive car - or, more precisely, for Mercedes to build a fully competitive engine for the first time since 1999.
Montoya's presence also gives the team a very positive profile as they look for new sponsors to cover the shortfall that will accompany the expected loss of tobacco advertising in 2007.
Williams bosses face a difficult few months looking for a new driver
BMW motorsport director Mario Theissen has tried to put a positive spin on Montoya's impending departure, pointing out that he is sure the Colombian will want to leave the team on a high.
But the fact remains that Williams have been left in an awkward position.
First of all, they face racing in 2004 with a driver they know is leaving the team at the end of the season - and one who will take many of their secrets with him to one of their biggest rivals.
As Williams technical director Patrick Head has admitted with masterly understatement, this will be "a bit odd".
Not only that, but Williams now have no drivers contracted beyond the end of 2004.
They are in discussions about extending Ralf Schumacher's contract.
Sharing the view of many in F1 that Schumacher is over-paid, Williams want to reduce the German's retainer.
But Montoya's departure strengthens the hand of Schumacher's manager Willi Weber as he seeks a rise from £7.5m to around £10.6m a year.
Either way, Williams need to sign at least one top driver for 2005, and will be trying to do so from a weak bargaining position - the drivers all know that Montoya has already left.
There are rumours that Williams will sign Jaguar's Mark Webber, but Renault have a prior option on the Australian and are expected to partner him with Alonso in 2005.
Coulthard will be driving for his future in 2004
McLaren's decision to sign Montoya will have come as little surprise to David Coulthard, and even his manager, Martin Brundle, has admitted it is "not great news" for the Scot.
Despite the deliberate vagueness of McLaren's announcement on Monday, no-one in F1 believes Coulthard has any chance of staying at the team for a 10th season in 2005.
That means that at the age of 34 he will be looking for a new job, with a not-altogether-convincing record behind him.
True, Coulthard has won 13 Grands Prix, but his critics argue that is a relatively small number for a man who has driven for one of the sport's top teams for his entire nine-and-a-half year career.
No-one would argue that, on his day, Coulthard can race with and beat anyone on the grid.
But he has not always been as consistent as he would have liked, an impression that was underlined by a series of poor qualifying performances in 2003.
Coulthard is driving to save his career next year, and he knows he will have to raise his game if he is to continue at the top of the sport.