Brazil 2014 marks the first time goal-line technology is used at the 32-nation football jamboree.
Fourteen cameras – seven focused on each goalmouth – will be used in all 12 World Cup stadiums in Brazil.
The cameras will record 500 images each second, and a computer will process the frames.
Within a second of a ball crossing the line, the referee’s special watch will vibrate and flash “GOAL.”
The small German start-up company GoalControl created the technology, which has been used for years in sports such as cricket, rugby, tennis and American-style football.
On the eve of the World Cup, the company demonstrated the goal-line technology at Brazil’s famous Maracana stadium in Rio de Janeiro, where the final will be played July 13th.
Officials from GoalControl and FIFA, world football’s governing body, say the system is 100% accurate.
During the demonstration at Maracana, GoalControl CEO Dirk Broichhausen mentioned a controversial incident at the 2010 World Cup in South Africa.
England’s Frank Lampard was denied a goal against Germany when his shot hit the bar and officials failed to spot the ball had bounced down just behind the line.
If it had been ruled a goal, England would have tied the Round of 16 match at 2-2, but Germany went on to win, 4-1.
The Lampard controversy is credited with helping persuade FIFA to adopt the goal-line technology.
FIFA officials say they’re sure they can trust the new technology, and more than 2,400 goal-line tests of the equipment have been carried out at several stadiums in Brazil ahead of the World Cup.