There is almost nothing the new American president does not want to change - policy, tone, foreign relations, the press pool. Mr Trump has told his cabinet nominees to be bold and be bold now.
He wants a shake-up of US government and he wants it soon. That is why his first 100 days will be so definitive. He has set the timetable for an ambitious agenda and in the next three months we will find out how much he can really shift.
There is a lot happening in Europe also during this 100 days. Britain is beginning the formal process of Brexit and the Dutch will hold elections which could herald the next step in the transatlantic populist march.
And of course, the French will gear up for their own election in which the National Front will be the focus of much attention. It is an extraordinary time on both sides of the Atlantic.
This exceptional moment demands examination and analysis. So the BBC is launching 100 Days, a daily programme that gives us the chance to look at these global shifts.
In many ways, the inauguration of Donald Trump marks the beginning of the test of the populist experiment. Now he owns the problems he campaigned against. Can his bold approach work, who will benefit and who won't and how will he engage with the rest of the world?
Every day for the next 100 days, with Christian Fraser in London and me in Washington, we will try to answer those questions.
As he left office, President Obama had a veiled warning for his successor - if you're going to try to change things and bring in bold ideas, make sure you're aware of the consequences. He also suggested that the weight of office would soon settle on Mr Trump's shoulders and cause him to look carefully and humbly at what he has taken on.
Mr Trump goes into the White House as the least popular incoming president on record. He won't like that. We know from his election campaign that he watches polls closely and however hard he tries to dismiss them as "phony" or "lying," they matter to him.
His low ratings today give him a powerful incentive to do better. That could mean a combination of both working on his tone (something which appears to be unpopular with large sections of the American public) and pushing hard with his agenda (much of which also seems to be popular with many Americans). That too, will make this a fascinating time.
Some of this is under Mr Trump's control, but some of it is not. The Republican Party will have a big impact in making his first 100 days successful - they can boost his legislative agenda or kill it.
The party owes Mr Trump a lot, he has just handed them Congress, the White House and the Supreme Court, that will win him a lot of favours. But many Conservatives do not agree with everything he wants to do and, if his poll numbers stay low, they will have less incentive to help him out.
So we have a busy, fascinating few months ahead of us. This populist trend is global and the test starts now. Mr Trump wants to change the look, feel and smell of Washington. Funny that, so did Mr Obama eight years ago.
100 Days, presented by Katty Kay and Christian Fraser, Monday - Thursday at 19:00 GMT on BBC News Channel and BBC Four and BBC World News at 19:00 GMT.

At least two of those who died were children, according to reports in local media.
A search operation is under way for people who are still missing.
The flash flood occurred at Cold Springs, near Payson, on Saturday afternoon, sweeping people down East Verde River.
The Payson Fire Department said that multiple forest fires in recent months had created piles of debris that burst down a creek and through the swimming hole.
But it was not raining in the area where people were swimming.
At least four people have been rescued from the water and treated for hypothermia.
The National Weather Service has issued a flash flood alert for much of Arizona until Monday evening, with more storms expected in the middle of next week.
Donald Trump has promised to take Washington by storm.
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A deadly flash flood sparked by monsoon-like rains has swept through a swimming hole in the US state of Arizona, killing at least eight people.