Ban Ki-moon has taken the "most impossible job in the world", as Trygve Lie famously said about the role of the Secretary-General. The Charter of the United Nations included the Secretariat among its principal organs, most certainly to grant some political prerogatives to the Secretary-General. According to the Charter, he is "the chief administrative officer of the Organization". It further stipulates that he "may bring to the attention of the Security Council any matter which in his opinion may threaten the maintenance of international peace and security".

In the early days of the United Nations, an atmosphere of extreme enthusiasm prevailed. Delegates were convinced they had adopted a system of collective security that would ban all wars forever, a feeling shared by Trygve Lie of Norway, who served as the first Secretary-General, from 1946 to 1952.

Dag Hammarskjöld of Sweden took over the post in 1953. By then, the international situation had begun to deteriorate and the earlier euphoria was replaced by an atmosphere of extreme tension between the East and the West. Because the Security Council was almost paralysed by repeated vetoes of the Soviet Union, Mr. Hammarskjöld often used quiet diplomacy, acting as a discreet mediator. On several occasions, he used the podium of the General Assembly to reply publicly to the attacks of Nikita Khrushchev of the USSR, who had advocated the replacement of the Secretary-General with three people: one from the East, one from the West and one from the South-the famous "troika".

U Thant of Burma (now Myanmar) was well known to the diplomatic community when he was appointed in November 1962 to replace Dag Hammarskjöld, who had perished with 15 collaborators in a plane crash on peace mission to Africa. A humble man, but with very firm convictions, U Thant promoted in the General Assembly and during his missions abroad his motto of the "Three Ds": decolonization, disarmament and development. He often used quiet diplomacy to calm lethal tensions during the difficult period of the cold war.

Kurt Waldheim of Austria had held a number of diplomatic functions, including as Permanent Representative to the United Nations and Minister for Foreign Affairs before serving as the fourth Secretary-General from 1972 to 1981. Throughout his mandate, he travelled to conflict areas of special concern to the United Nations.

The appointment of Javier Pérez de Cuéllar of Peru came as a complete surprise to him as he was just beginning to enjoy retirement. He knew the United Nations inside out since he had been Permanent Representative and Under-Secretary-General for Special Political Affairs. Mr. Pérez de Cuéllar contributed significantly to resolving the conflicts in Central America and defusing the war between Iran and Iraq. During his ten-year mandate as Secretary-General, from 1982 to 1991, a number of sources of tension associated with the cold war were gradually eliminated.

Boutros Boutros-Ghali of Egypt served from 1992 to 1996. Before his appointment as Secretary-General, he had held some important functions, including as Minister of State for Foreign Affairs, and as a negotiator of the 1979 Camp David Accords between Egypt and Israel.
Before becoming Secretary-General in 1997, Kofi Annan of Ghana already had 35 years of UN experience, joining the UN system in 1962. His first major initiative as Secretary-General was his plan for reform, "Renewing the United Nations". In 2000, he issued a report that served as a basis for the Millennium Declaration, adopted by Heads of State or Government in September 2000.

Ban Ki-moon of the Republic of Korea has ties with the United Nations dating back to 1975. On his appointment as the eighth Secretary-General, he was Minister for Foreign Affairs. When he took the oath of office on 14 December 2006, he created a precedent by placing his left hand on the UN Charter. Mr. Ban said: "By strengthening the three pillars of the United Nations-security, development and human rights-we can build a more peaceful, more prosperous and more just world for our succeeding generations. As we pursue our collective endeavour to reach that goal, my first priority will be to restore trust. I will seek to act as a harmonizer and bridge builder. And I hope to become known to all of you-Member States or Secretariat-as a Secretary-General who is accessible, hard-working and prepared to listen attentively."