The Secretary-General of the United Nations is a unique figure in world politics. At once civil servant and the world's diplomat, lackey of the UN Security Council and commander-in-chief of up to 100,000 peacekeepers, he or she depends on states for both the legitimacy and resources that enable the United Nations to function. The tension between these roles – of being secretary or general – has challenged every incumbent. The first, the Norwegian Trygve Lie (1946–1952), memorably welcomed his successor to New York's Idlewild Airport with the words: “You are about to enter the most impossible job on this earth.”

The formal responsibilities of the job are few and ambiguous. The UN Charter defines the position as “chief administrative officer” of the United Nations Organization, a capacity in which he or she serves the Security Council, the General Assembly, and the Economic and Social Council, as well as performing “such other functions as are entrusted to him by these organs.” At the same time, the Secretary-General is granted significant institutional and personal independence: the Secretariat he or she leads is itself a principal organ of the United Nations; the Secretary-General and the staff serve as international officials responsible only to the organization; and the Secretary-General is given a wide discretion to bring to the attention of the Security Council “any matter which in his opinion may threaten the maintenance of international peace and security.”

The manner in which these responsibilities have been fulfilled over the past six decades has depended as much on politics as it has on personality.