Ambassadors of the United States are persons nominated by the president to serve as the country's diplomatic representatives to foreign nations, international organizations, and as ambassadors-at-large. Under Article II, Section 2 of the U.S. Constitution, their appointment must be confirmed by the United States Senate; while an ambassador may be appointed during a recess, they can serve only until the end of the next session of Congress, unless subsequently confirmed.
Ambassadors are the highest-ranking diplomats of the U.S. and are usually based at the embassy in the host country. They are under the jurisdiction of the Department of State and answer directly to the secretary of state; however, ambassadors serve "at the pleasure of the President", meaning they can be dismissed at any time. Appointments change regularly for various reasons, such as reassignment or retirement.
An ambassador may be a career Foreign Service Officer (career diplomat – CD) or a political appointee (PA). In most cases, career foreign service officers serve a tour of approximately three years per ambassadorship, whereas political appointees customarily tender their resignations upon the inauguration of a new president.
The State Department publishes a monthly list of ambassadors. A listing by country of past chiefs of mission is maintained by the Office of the Historian of the U.S. Department of State, along with the names and appointment dates of past and present ambassadors-at-large and mission to international organizations.