South Korea's Ban Ki Moon has taken over as secretary general of the United Nations - the first Asian to hold the post in 35 years.
Ban Ki-moon wants to a bridge-builder and harmoniser
The South Korean former foreign minister is the organisation's eighth leader and succeeds Kofi Annan, who was at the helm for 10 years.
Mr Ban faces a number of global crises as well as reform of the UN itself.
He has vowed to be a bridge-builder and a harmoniser and to set the highest ethical standards.
The BBC's UN correspondent in New York, Laura Trevelyan, says Mr Ban, 62, has given little away about his diplomatic priorities.
One key challenge the new secretary general inherits from Mr Annan is trying to get the Sudanese government to accept a joint UN-African Union peacekeeping force in Darfur.
Mr Ban will also hope to bring his regional experience to the thorny question of North Korea, our correspondent adds.
The US is leading a drive for greater reform of the UN and analysts say Washington will expect Mr Ban to deliver, having strongly backed his candidacy.
White House spokesman Scott Stanzel said on Sunday: "The United States will rely on his leadership to help steer the UN organisation through the reforms already underway, and to propel it even further on the path of reform."
Another key challenge will be the question of whether to extend the 15-member Security Council.
Mr Ban has named India's Vijay Nambiar as his cabinet chief.