Illegal substitution is a penalty in football that occurs when a player fails to make a substitution and the team has too many players on the field as a result. This can happen in a few different ways. On offense, an illegal substitution penalty will be called if there are more than 11 players in the huddle at any point. On offense or defense, having 12 or more players lined up on the field when the ball is snapped also constitutes an illegal substitution (or a “too many men on the field” penalty) . The penalty for an illegal substitution is a loss of 5 yards and the down being replayed. If the penalty is called on the defense, the offense will have the option to decline the penalty and instead take the result of the play rather than 5 yards.

Most illegal substitution penalties occur when there is some confusion over who is supposed to leave the field of play during a change of personnel or formation. On offense, a player may not leave the huddle because they are unaware that they are being substituted for. The quarterback can usually help with this by standing away from the huddle until substitutions are completed. On defense, a player may be unaware that there is a personnel change and forget to sub out for a different player.

It is important to note that illegal participation is a different penalty from illegal substitution. Illegal participation occurs when a team has 12 players on the field while a play goes as normal. This penalty is more severe than illegal substitution, with a 15-yard penalty being assessed.

In summary, illegal substitution is a penalty in football that occurs when a team has too many players on the field due to a player failing to make a substitution. The penalty for this infraction is a loss of 5 yards and the down being replayed.