Ban Ki-moon of South Korea takes over as UN secretary general Monday, facing numerous crises across the globe as well as the challenging task of reforming the United Nations itself.
The 62-year-old diplomat's handling of the reform, launched in 2005 by his predecessor Kofi Annan, will be closely watched by UN members, notably the most powerful, the United States, which strongly backed his candidacy.
Washington and others want the UN to become more efficient and transparent and be stricter over ethics and management following a series of scandals, including in the corruption-tainted Iraq oil-for-food program.
Aware of the huge task ahead of him, Ban said in a news conference in December that he wanted to "restore trust" in the United Nations.
In subsequent interviews, he promised to devote most of his attention to solving the immediate problem of regional conflicts around the world.
He said he wanted to be a "harmoniser" and a "bridge-builder," pointing out that the conflict in the Middle East, the Lebanese situation and the crisis in Darfur would be among his priorities.
White House spokesman Scott Stanzel said on Sunday that Ban assumes his new role at a time of great challenge and opportunity at the United Nations.
"The United States will rely on his leadership to help steer the UN Organization through the reforms already underway, and to propel the Organization even further on the path of reform," Stanzel noted.
Another challenge for the former South Korean foreign minister is the enlargement of the 15-member Security Council, whose composition, member states generally agree, reflects the realities of 1945 rather than of the 21st century.
But no enlargement plan has been approved due to disagreements among member states, notably the council's five permanent, veto-wielding members: the United States, France, Britain, Russia and China.
While Ban tackles a challenging chore within the world body, he also faces a slew of conflicts and crises around the globe.
The gloomy list includes the war in Iraq, the Israeli-Palestinian conflict, the bloodshed in Sudan's Darfur region and the disputed nuclear programs of North Korea and Iran.
The UN oversees more than 100,000 blue-helmeted international troops from its New York headquarters. Peacekeepers are scattered across the world, from the Democratic Republic of Congo to Kosovo and Haiti.
The new UN chief's job also includes numerous other crucial issues of international concern, including the fight against AIDS, protecting children in conflict zones, poverty and global warming.
"If we look at middle, long-term affairs, we must take urgent action to address this climate change issue otherwise our future generations will suffer from this issues," Ban told the BBC.
As Ban takes over, UN members eagerly await his decisions on key leadership posts in the world body.
On Sunday, he named Vijay Nambiar of India as his cabinet chief and Michele Montas as his spokeswoman and said he planned to make more appointments in the coming days.
Nambiar was a special adviser to Annan and has also served as India's UN envoy. Montas was a Haitian journalist who heads the French unit of UN Radio.
"Sunday's appointments will serve as a solid basis for establishing my team and pursuing a program of reform of the Secretariat to provide continuity along with change," Ban said in a statement.