Ambassador - Ambassador Extraordinary and Plenipotentiary

Among European powers, the ambassador extraordinary and plenipotentiary (French ambassadeur extraordinaire et plénipotentiaire or German außerordentlicher und bevollmächtigter Botschafter) was historically deemed the personal representative of the Sovereign, and the custom of dispatching ambassadors to the head of state rather than the government has persisted. For example, ambassadors to and from the United Kingdom are accredited to or from the Royal Court of St. James's (referring to St. James's Palace).

Because members of the Commonwealth of Nations have or had a common head of state, they do not exchange ambassadors, but instead have High Commissioners, who represent the government, rather than the head of state. In diplomatic usage, high commissioner is considered an equivalent rank and role to an ambassador.

While the title generally reflects the ambassador's second head position as head of a diplomatic mission, in some countries the term may also represent a rank held by career diplomats, as a matter of internal promotion, regardless of the posting, and in many national careers it is quite common for them to be appointed to other functions, especially within the ministry/ministries in charge of foreign affairs, in some countries in systematic alternation with actual postings.

The formal form of address for an ambassador is generally the form that would be used to address a head of state: "(Your/His/Her) Excellency" followed by name and/or the country represented. In many countries, less formal variations are frequently used, such as "Ambassador" followed by name, or the name followed by "Ambassador of...". In the United States, "Mister/Madam Ambassador" may be used.

In some countries, a former ambassador may continue to be styled and addressed as ambassador throughout their life (in the United States, "Mr. Ambassador" or "Madam Ambassador" may be heard). In other countries, ambassador is a title that accrues to its holder only with respect to a specific position, and may not be used after leaving or beyond the position. Some countries do not use the term while an ambassador is in the home country, as the office holder is not an ambassador there; for example, a Canadian ambassador while in Canada is not generally addressed as ambassador, although he or she may be referred to as "Canadian ambassador to ..."; that is, with reference to a specific job function; the person is addressed or styled as ambassador only while holding such office.

Ranking below full ambassador are the rank of Envoy, Minister Resident and Chargé d'affaires. They represent their government rather than their head of state. For further details, see diplomatic rank.