First Lieutenant is a military rank.
The rank of Lieutenant has different meanings in different military formations (see comparative military ranks), but the majority of cases it is common for it to be sub-divided into a senior (First Lieutenant) and junior (Second Lieutenant) rank. In navies it may relate to a particular post rather than a rank.
Before 1871, when the whole British Army switched to using the current rank of "Lieutenant", the Royal Artillery, Royal Engineers and Fusilier regiments used "First Lieutenant" and "Second Lieutenant".
Historically the lieutenants in a ship were ranked in accordance with seniority, with the most senior being termed the First Lieutenant and acting as the second-in-command, unless the ship was complemented with a Commander. Although lieutenants are no longer ranked by seniority, the post of "First Lieutenant" remains. In Minor War Vessels, Destroyers and Frigates the First Lieutenant is second in command, Executive Officer (XO) and head of the executive branch; in larger ships where a Commander of the warfare specialisation is appointed as the Executive Officer, a First Lieutenant is appointed as his deputy. The post of First Lieutenant in a shore establishment carries a similar responsibility to the First Lieutenant of a Capital Ship.
In the U.S. Army, Marine Corps, and Air Force, a First Lieutenant (O-2) is the second-lowest ranking commissioned officer. It is one grade above the rank of Second Lieutenant (O-1) and one grade below a Captain (O-3). Second Lieutenants are usually promoted to First Lieutenant after 18 months (Army) or 24 months (Air Force and Marine Corps) of commissioned service.
The difference between the two ranks of Lieutenant is slight, primarily being experience and higher pay. It is not uncommon to see officers moved to positions requiring more experience after promotion to First Lieutenant. For example, in the Army these positions can include leading a specialty platoon, or assignment as the Executive Officer for a company-sized unit (65-150 soldiers). In the Air Force, a First Lieutenant may be a flight commander, although in the operations group a First Lieutenant is generally a pilot fresh out of undergraduate pilot training with few supervisory responsibilities.
For other countries, the equivalent rank to a US Army First Lieutenant (O-2) is listed below.