A player is offside if they are closer to the opponent's goal line than both the ball and the second-last defender at the moment a teammate passes the ball to them. Being in an offside position is not an offense by itself — the player must be actively involved in play (touching the ball, interfering with an opponent, or gaining an advantage).

1. Position: The attacking player is in the opponent's half and closer to the goal line than the second-last defender (usually the last outfield player, since the goalkeeper is typically the last).
2. Timing: The position is judged at the exact moment the ball is played by a teammate — not when the player receives it.
3. Involvement: The player must actively participate — by playing the ball, blocking a defender's view, or gaining an advantage from a rebound.

The 2026 World Cup uses Semi-Automated Offside Technology (SAOT). Cameras track player limb positions 50 times per second. When a potential offside occurs, the system generates a 3D animation within seconds, helping the referee make accurate calls faster than ever. This technology debuted at the 2022 World Cup in Qatar.

VAR (Video Assistant Referee) still reviews all goal-scoring situations, including offside.

Without the offside rule, attackers could stand next to the opposing goalkeeper and wait for long passes. The rule forces attacking players to time their runs and stay in line with defenders, making the game more dynamic and tactically interesting.