That is the question on everyone's lips after the 2008 world champion signed a three-year deal with the Silver Arrows.
Hamilton has been developed by McLaren since the age of 13, and until this season it was unthinkable that he would leave the Woking squad.
However contract negotiations have been a long-running saga throughout the present campaign and he has ultimately opted to make the move to Ross Brawn's team.
"Mercedes-Benz has such an incredible heritage in motorsport, along with a passion for winning which I share," said Hamilton.
"I believe that I can help steer the Silver Arrows to the top and achieve our joint ambitions of winning the world championships."
Brawn echoed those sentiments: "Over the past three years, we have been putting in place the foundations and building blocks that are needed to compete regularly for the world championship.
"The potential is now there to match any other team on the grid."
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Mercedes took over champions Brawn GP at the end of 2009 but have had only one victory - Rosberg's success in this year's Chinese Grand Prix - to celebrate.
While that was the first by a works Mercedes since 1955, McLaren are regular challengers for the title and have won five times this year - three of them provided by Hamilton.
On paper, Hamilton is swapping a place with proven winners for a British-based team with little success but plenty of potential. Mercedes also make the engines used by McLaren and have a substantial development budget.
And Brawn has a highly impressive track record after multiple championships as technical director at Benetton and Ferrari and boss of the Brawn team he saved after previous owners Honda pulled out in late 2008.
- Ross Brawn
- Lewis Hamilton
- Silver Arrows