In football, the official exchange of one active player on the field for another.
The primary responsibility for substitutions in football lies with coaches and players. By rule, teams can only field eleven players for any given play. During a substitution, the substituting player typically enters the field of play as the substituted player leaves. Substitutions are either planned ahead of time or called by coaches when they occur.
An illegal substitution penalty or substitution infraction is called by officials when one of the following occurs: there are twelve or more players in the huddle for three to five seconds, in the formation, or on the field of play when the ball is snapped. The penalty for illegal substitution is five yards.