A team must have 12 players on its active roster, although they can drop to 11 for up to two weeks at a time.  They must suit up at least eight players for every game.  Any remaining players must be on its Inactive List, and are ineligible to play in games.  A team must have a minimum of one and a maximum of three players on its Inactive List, although they can drop to zero for up to two weeks at a time, and can temporarily have four with league approval in the event of a hardship.  The composition of the Inactive List can change on a game-by-game basis -- no less than 60 minutes prior to tipoff, the team must present to the official scorer a list of the players who will be active for that game.  A player can be inactive for as little as one game.  While individual teams are only required to carry 13 players (12 active and one inactive), the NBA also guarantees a league-wide average of at least 14 players per team.  The league is surcharged if they do not meet this obligation.

Injured Reserve is the previous name for the Inactive List.  It was originally intended for players who were injured and unable to play, however teams often used it as a convenient place to stash extra players.  While a medical reason was required for players to be put on Injured Reserve, the league did not insist on an independent physician confirming the diagnosis. Thus it was common for a seemingly healthy player to suddenly develop "back spasms" right before rosters were cut to 12 players, and spend the entire season on Injured Reserve as a result.  With the current CBA they gave up the ghost, dropped the medical requirement, and changed "injured" to "inactive."

Players assigned to the NBA Developmental League (see question number 66) are automatically placed on their team's Inactive List.