The rank of Lieutenant has different meanings in different military formations (see comparative military ranks), but in all cases it is common for it to be divided.

In the United States Army, Air Force and Marine Corps, First Lieutenant is the second-lowest ranking commissioned officer. It is one step above the rank of Second Lieutenant. In the British Army and Royal Marines, the rank is only Lieutenant, with no ordinal attached.

In the Royal Navy, the rank of Lieutenant is equivalent to that of a Captain in the army. First Lieutenant is not a specific rank, but aboard a ship, one officer would be designated as First Lieutenant (familiarly, "Number One") and would be the effective second in command. This is still the case, although the First Lieutenant often now holds the rank of Lieutenant Commander. In smaller ships he also holds the position of Executive Officer, although in larger ships he is the XO's deputy.

In the United States Navy, First Lieutenant is a position title and is held by the officer in command of the Deck Department. On smaller ships, a First Lieutenant holds the rank of Lieutenant Junior Grade. 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.