First Lieutenant is a junior, company grade commissioned officer rank of the UNSC Marine Corps, Army, and Air Force, equal in rank to Lieutenant, Junior Grade in the UNSC Navy. They usually serve as the XO (executive officer) of a Company, but can be given command of platoons, such as the case of First Lieutenant Melissa McKay in Halo: The Flood.

A Second Lieutenant usually receives a promotion to First Lieutenant after 18 to 24 months service.

In informal settings, First Lieutenants are referred to simply as "Lieutenant" or "LT."

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