The short answer
Goal-line technology has one legal job: determine whether the whole ball crossed the goal line between the posts and under the crossbar. An approved system must send an immediate automatic signal—within one second—only to the match officials, normally through the referee's watch, headset or both. It does not award the goal by itself or decide fouls, offside and other incidents. Competition rules must authorise its use and the referee must test it before the match.
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The legal test is the whole ball
IFAB Law 10 awards a goal only when the whole ball passes over the whole goal line between the posts and under the crossbar, provided the attacking team has committed no offence.
Goal-line technology measures the geometric part of that test. A ball touching the vertical plane of the line has not fully crossed.
The system has only one permitted purpose
Law 1 says GLT applies solely to the goal line and only to determine whether a goal has been scored. It cannot decide handball, offside, fouls or which team should restart.
Those limits separate it from VAR and semi-automated offside, even when the technologies coexist in one stadium.
The signal must arrive privately and quickly
An approved system automatically confirms the result within one second. The notification goes only to match officials, typically by vibration and a visual message on the referee’s watch and, where permitted, through a headset.
The crowd graphic is not the legal signal. It is a broadcast or stadium explanation after the official receives the information.
Competition rules must authorise its use
GLT is optional, not a universal requirement of every football match. The competition must provide for it and organisers must use a system that meets the FIFA Quality Programme.
That is why the Laws of football remain playable at fields with no electronic system. Technology assists the referee where adopted; it does not redefine a goal.
The referee tests it before the match
The referee must perform the prescribed functionality test. If the equipment does not work according to the testing manual, it cannot be used and the matter must be reported.
A failed system therefore does not suspend the whole of Law 10. Officials revert to ordinary observation and authorised match-official assistance.
Detection does not excuse an attacking offence
GLT can correctly report that the ball crossed while a goal is still disallowed for an earlier infringement. The system answers where the ball went, not whether the scoring sequence was legal.
This is the central distinction: measurement supplies one fact; the referee applies all relevant laws. Football technology is most reliable when its jurisdiction is kept narrow.
Frequently asked questions
What does goal-line technology decide?
Only whether the whole ball crossed the goal line inside the goal.
How quickly must the signal arrive?
IFAB requires immediate automatic confirmation within one second.
Who receives the signal?
Only the match officials, through the referee's watch, visual or vibration signal, headset, or approved combination.
Is GLT the same as VAR?
No. GLT is automatic and limited to the goal line; VAR reviews specified categories using video.
Can every competition use it?
Only when competition rules authorise it and the system meets the FIFA Quality Programme requirements.
What if the system fails before kickoff?
The referee must not use a system that fails the required pre-match functionality test and must report the issue.