In the United States, second lieutenant is the normal entry-level rank for most commissioned officers in the Army, Air Force and Marine Corps and is equivalent to the rank of ensign in the Navy and Coast Guard.

In the Army and Marine Corps, a second lieutenant typically leads a platoon-size element (16 to 44 soldiers or Marines). In the army, until December 1917, the rank bore no insignia other than a brown sleeve braid on blouses and an officer's cap device and hat cord. In December 1917, a gold-colored bar similar to the silver-colored bar of a first lieutenant was introduced. In US military slang, the rank is sometimes called "butterbar" in reference to the insignia.^[10]

In the Air Force, depending upon the career field, a second lieutenant (2nd Lt) may supervise flights (of varying sizes) as a flight leader or deputy flight leader, or may work in a variety of administrative positions at the squadron, group, or wing level. A significant number of Air Force second lieutenants are full-time flight students in training for eventual designation as USAF pilots, combat systems officers or air battle managers.

In order to become a commissioned officer in all branches, one must complete specialized officer training. In the Army, this is known as officer candidate school (OCS). The Army is the only branch in which the individual must go through enlisted basic training before becoming an officer candidate and attending OCS