First Lieutenant

==United States==
===US Army, US Marine Corps and US Air Force===
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.
===US Navy and US Coast Guard===
In the U.S. Navy and Coast Guard, First Lieutenant is a position title, instead of a rank. It is held by the officer in command of the deck department. On smaller ships, the First Lieutenant holds the rank of Lieutenant, junior grade or Ensign. On larger vessels, the position is held by a Lieutenant or, in the case of extremely large warships such as aircraft carriers, a Lieutenant Commander or even full Commander. However, on submarines, where the deck department may only have a few junior sailors, the First Lieutenant may be a senior enlisted member, such as a First-Class Petty Officer or Chief Petty Officer.