Under Article 14 of the 1961 Vienna Convention on Diplomatic Relations (VCDR), heads of mission are divided into three classes: (a) ambassadors or nuncios accredited to heads of state, and other heads of mission of equivalent rank; (b) envoys, ministers, and internuncios accredited to heads of state; and (c) chargés d'affaires accredited to ministers for foreign affairs. The first two classes typically lead embassies; chargés d'affaires en pied head smaller missions, while chargés d'affaires ad interim run an embassy temporarily when the ambassador is absent or the post is vacant.
A head of mission's functions, set out in Article 3 VCDR, include representing the sending state, protecting its interests and those of its nationals, negotiating with the receiving government, reporting on conditions in the host country, and promoting friendly relations. Appointment requires agrément: the receiving state must consent before accreditation (Article 4), and may at any time declare the head of mission persona non grata without explanation (Article 9).
The head of mission formally assumes functions upon presenting credentials (lettres de créance) to the head of state of the receiving country, or in the case of a chargé d'affaires, upon notification to the foreign ministry. Precedence within the diplomatic corps is determined by class and by date of taking up functions (Article 16). In many capitals the longest-serving ambassador serves as dean (doyen) of the corps; in Catholic-tradition states, the Apostolic Nuncio holds this role by custom.
Heads of mission enjoy the fullest range of diplomatic privileges and immunities under the VCDR, including inviolability of person, immunity from criminal jurisdiction, and exemption from most taxation and customs duties.
Example
When Linda Thomas-Greenfield presented her credentials in 2021, she became head of the U.S. Mission to the United Nations with the rank of ambassador.