An executive officer, also known as a first officer, is the second-in-command of a starship, starbase, or installation.
In Starfleet, this position is almost always held by an officer with several years of experience. On the largest ships in Starfleet, such as battleships, battlecruisers, and heavy cruisers, this position is almost always held by someone with the rank of Commander or Lieutenant Commander. On ships of moderate size, such as a light cruiser, destroyer, or frigate, the position is commonly held by an officer at the rank of Lieutenant Commander or Lieutenant. Starfleet's smallest ships, like corvettes and and transports, often have a first officer at the rank of Lieutenant or Lieutenant (JG).
Starfleet regulations allow for the first officer to assume command in the event the Commanding Officer dies or is otherwise incapacitated. Traditionally, this officer is responsible for implementing the orders of the commanding officer, conducting crew evaluations, and dealing with crew management issues. He or she may also be expected to be the standing away team leader. Some Starfleet Captains, such as Aron Amadeus, also add the duty of Training Officer to this individual.