Aboard a starship the first officer was the second-in-command, just below the captain. On a Federation starship, the first officer had the following functions:

• the principal adviser to the captain
• implementer of the captain's orders
• preparation and issue of duty rosters (VOY: "The Gift", "Tsunkatse", "Endgame")
• 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 (TNG: "Lower Decks")