Why it arrived when it did
Football resisted goal-line technology for years on the argument that the game should be the same at every level. The 2010 World Cup in South Africa changed the politics: a shot for England against Germany in Bloemfontein crossed the line by a visible margin on television and was not given. Testing under the FIFA Quality Programme followed, and in July 2012 the International Football Association Board approved goal-line technology for use in the Laws. Two systems were certified at that stage: Hawk-Eye, which uses cameras, and GoalRef, a magnetic system developed with the Fraunhofer Institute in Germany.
The FIFA Club World Cup in December 2012 was the first FIFA tournament to use it. The Premier League installed Hawk-Eye for the 2013–14 season. The 2014 World Cup in Brazil used GoalControl-4D, another camera system. Bundesliga clubs voted against adoption in 2014, then reversed that decision and introduced Hawk-Eye for 2015–16, the same season Serie A adopted it. UEFA brought it into its competitions from Euro 2016 onwards.
How the camera systems work
A camera installation places seven high-speed cameras on each goal, fourteen in total, mounted in the stadium roof or on the stands so that they look down at the goalmouth from different angles. Each frame from each camera gives the ball's position in that picture; combining the cameras gives its position in space, using the same triangulation described on the Hawk-Eye sheet. Because the ball's diameter is known, the system models the whole sphere, not just its centre, and "wholly over the line" becomes a straightforward geometric test.
When the test is passed, an encrypted signal goes to a watch worn by each match official: it vibrates and displays the word GOAL. The FIFA Quality Programme requires that signal to arrive within one second of the ball crossing. The referee sees no graphic during play. The animation viewers see is generated afterwards for the broadcaster, which is worth remembering when it looks more certain than the referee did.
The magnetic alternative
GoalRef, and before it the Cairos system developed with adidas, took a different route. A passive coil sits inside the ball; antennas in the goal frame generate a low-frequency magnetic field; when the ball passes through the plane of the goal the field changes in a way the electronics recognise. Occlusion is irrelevant, because nothing has to see the ball. The costs are that the match ball has to be a special ball approved for the purpose, and that the system produces no picture. Camera systems won commercially in large part because they generate a graphic that broadcasters can show, not because the physics was better.
Working note
The FIFA Quality Programme's testing manual for goal-line technology specifies the positional tolerance and the one-second time limit, and requires both laboratory tests and in-stadium tests, including shots at speed and deliberate obstruction of cameras. The exact tolerance is in that manual; quoting it from memory is how wrong numbers spread.
What it cannot see
A camera system fails when no camera can see the ball, and in June 2020, in the first Premier League match after the pandemic restart, that happened. At Villa Park against Sheffield United the goalkeeper carried the ball behind his own line with several players packed around him, all seven cameras were blocked, no signal was sent and no goal was given. VAR did not intervene because the protocol treats the goal-line system as decisive. Hawk-Eye apologised and described the level of occlusion as unprecedented in its installations. The case is useful precisely because it is rare: the system did not misjudge the position, it had no position to judge.
The second limit is more fundamental. Goal-line technology says nothing about whether the goal should count. Offside, a foul or handball in the build-up remain matters for the referee and, where present, VAR. A GOAL signal on the watch and a disallowed goal can coexist without either being wrong.
Cost, coverage and the alternatives to having it
A certified installation is a per-stadium expense, plus ongoing testing to keep certification, and that is why most of football does not have it. Where it is absent, competitions that have VAR review goal-line incidents from broadcast angles, which is slower and rarely as clear, because broadcast cameras were not placed to look along the line. Ligue 1 dropped goal-line technology in 2018 after disputes with its provider, relying on VAR instead, which shows the trade in the other direction: a league can decide the marginal certainty is not worth the fee.
The 2022 World Cup added a connected ball with an inertial sensor sampling 500 times a second, but that fed the semi-automated offside system, detecting the moment of the kick; goal decisions stayed with the camera system. Sensor balls may one day replace goal-line cameras, but that would return football to the magnetic approach's old constraint, a mandated special ball, so it is not obviously progress.