The offside rule is one of the oldest football rules, but is still a much discussed rule. This is probably because of the relatively complicated set of provisions provided by the sport's law-making body, the IFAB for deciding if a player in an offside position is actually guilty of an offside offence by becoming involved in active play. It can also be a difficult decision for the referee or assistant referee to make as they need to watch the ball, the player playing the ball and also the attacker who is seeking to receive the ball at the same time.

Offside is an offence committed by the team which has the ball and is punished with an indirect free kick. It is a common misconception that the ball must be played forward for an offside offence to be committed. This is incorrect however, as an offside offence is related to the position of the player in relation to the last two opponents, the ball and the opponent's goal line rather than the direction the ball is played. If the player is closer to the opponent's goal line than both the ball and the second-last opponent when it is played by a team mate, she or he is in an offside position.

But there are some exceptions:

• Players cannot be offside in their own half of the field.
• It is not possible to be offside from a throw-in, corner kick or goal kick.
• If the ball is deliberately played by a member of the opposing team, an attacker cannot be offside.
• A player who is behind the other team's goal line is considered to be on the goal line for the purposes of offside. It is an offence to leave or re-enter the field of play without permission but not if it is part of a natural playing movement.
• If the player is behind the ball when it is played, he or she cannot be offside.
• An attacker who is exactly in line with the 2nd last defender or the ball is not considered to be offside.
• If a player does not become involved in active play, then it is not an offside offence even if that player is in an offside position (the concept of passive offside introduced in 1924). A player can become involved in active play by touching the ball (either directly from a team mate's touch or after it rebounds off the goal frame or an opponent) or interfering with another player's ability to play the ball.