Barack Obama has begun his presidency by promising to tackle the many serious challenges which face the United States.
He took the oath of office in front of around two million people on the steps of the Capitol Building in the heart of Washington.
BBC News website readers have sent in their videos showing how they are marking the event.
CAMERON FURE, 26, PHOTOGRAPHER, MICHIGAN
Cameron Fure says he is proud that Barack Obama is his president
This is not Cameron's first inauguration experience - he says he was 'shocked and awed' when he saw Bill Clinton being sworn in in 1993.
"I am thrilled at the prospect of having a new president who ,not only Americans can be proud of, but a president the whole world can be proud of," he said.
PAUL BROOKS, 32, FINANCIAL CONSULTANT, WASHINGTON
Paul Brooks took this video on the train from Rosslyn, Virginia
Paul, who is originally from Cardiff, Wales, has lived in the US since 1999.
He saw these celebratory scenes on the train from Rosslyn metro station, Arlington, Virginia, before the inauguration.
He said: "I hadn't seen anything like it, and had to capture the moment."
MARK NEALEIGH, 24, SHOP WORKER, NEBRASKA
Mark Nealeigh, 24, from Nebraska, has been planning this trip for six months.
He said: "I got on an AmTrak train in Omaha, Nebraska. This was my first time on a train so it really added to the whole experience - being able to watch as more than half of the country passed by my windows put me in an interesting frame of mind as I neared the capitol."
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