- The principal adviser to the captain
- Implementer of the captain's orders
- Preparation and issue of duty rosters
- Commander of the vessel in the event of the captain's absence, incapacitation or death
- Work with the chief medical officer or counselor on crew evaluations
In Starfleet the position is usually held by an officer with the rank of commander or lieutenant commander, or more rarely lieutenant. The executive officer's rank is never permitted to exceed that of the captain, though on rare occasions Starfleet has permitted an experienced officer a rank equal to that of the ship's captain. The position is also referred to as the first officer or the XO.
In Starfleet's early period of existence, the first officer would accompany the captain on landing parties, depending on the situation. By the 24th century, however, it has become standard that the first officer leads most away teams, while the captain remains on-board the ship.