It might be the most intimidating rule to learn as a new fan of the game. Even for the diehards, it inspires heated debates on a near-weekly basis. That's right: it's the offside rule.
With the 2026 FIFA World Cup taking place this summer, there's no better time for a refresher on how it all works.
Here's what you need to know about offside, why it exists and how it will be implemented at the World Cup.
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What is offside?
Let's start with the basics. A player is in an offside position when they are in the opponents' half of the pitch and closer to the opponents' goal than both the ball and the second-to-last defender. (Why second-to-last? To exclude the goalkeeper, who in the vast majority of cases is the last defender.)
It's not wrong to be standing in an offside position in general. But when a player receives a pass from a teammate and is in an offside position the moment the ball is played, they are flagged by the assistant referee for an offside offense, and the opposing team gets an indirect free kick.
It's important to emphasize the point about timing. The player must be onside at the moment the ball is played, but immediately after that, they are allowed to run into an offside position to gain possession of the ball.
Why does this rule exist?
If players could stand wherever they wanted, you'd see many players hovering around the opponent's goal, waiting for the ball to reach them so they could score with ease. There's a word for this -- cherry-picking -- and it doesn't make for a very entertaining sport to watch.
The offside rule creates the need for skillful players and smart tactics. Without it, soccer would be unrecognizable compared with the game we know today.
What makes it so complicated?
The devil's in the details when it comes to offside, and here are a few of the most important complications and exceptions.
Parts of the body. A player's hands and arms aren't relevant when it comes to offside. Why? Because those parts of the body can't be used to score a goal.
Interfering with play. A player is penalized for offside only if they are involved in the play. That includes not just receiving a pass, but also challenging an opponent for the ball or blocking an opponent's line of sight. This aspect of the law can be highly subjective and lead to contentious arguments.
Location on the pitch. A player can't be offside in their own half of the pitch.
Play restarts. A player can't be offside if they receive the ball directly (without any touches/passes in between) on throw-ins, goal kicks or corner kicks. They can, however, be offside on free kicks awarded for fouls and other violations.
Deflections. If a pass deflects off an opponent before it arrives at a player in an offside position, that player is still considered offside. But if the opponent makes a "deliberate play" on the ball -- e.g. an intentional pass or clearance -- and the ball reaches the player in an offside position, they are not penalized for being offside and play can continue. One way this happens is when a defender tries to pass the ball back to their goalkeeper, but fails to spot an opposing forward who swoops in and wins the ball. That forward would not be penalized for offside.
How will it be enforced at the World Cup?
When a player is ruled offside, one of the assistant referees (located on the sidelines) will raise their flag to indicate the call to the referee.
The institution of the video assistant referee (VAR) has changed the way offside can be enforced. Essentially, the VAR is an additional referee situated in a control room, who reviews calls made by the referee on the pitch using video screens. The use of replay allows offside calls with much less margin to be made (though it still has its limitations).
At the World Cup, the VAR will be aided by semi-automated offside technology (SAOT), which has already been implemented for the 2022 World Cup, several editions of the UEFA Champions League, and other major competitions -- and we'll see it again this summer.
SAOT enables key steps of the offside-checking process to be automated and sped up, making for faster and more accurate calls. However, more complicated cases must still be reviewed thoroughly by the VAR.