Did the ball cross the line? It's a question soccer fans have been asking at least since the 1966 World Cup final, when England striker Geoff Hurst's extra time goal against Germany was given by a Swiss referee on the advice of his Soviet linesman.
Soccer's world governing body has been trying out two systems to check close calls, and will put them to the tournament test at the Club World Cup, which starts next weekend in Japan.
The aim is to use the most accurate version at the 2014 World Cup in Brazil.
A last-16 World Cup match two years ago in South Africa went a long way to convincing Fifa the technology was needed after England's Frank Lampard had what appeared to be a perfectly good goal ruled out against Germany.
The decision presaged a 4-1 loss for Lampard's side, prompting authorities to accept they must do more to prevent such miscarriages of sporting justice.
Two systems - one German, one English - have been certified by Fifa.
One is GoalRef, which uses magnetic fields and a modified ball. The other, Hawk-Eye, uses cameras in similar fashion to tennis and cricket.
GoalRef is the brainchild of researchers Fraunhofer Institute for Integrated Circuit in Germany, and Hawk-Eye is a British system.
"At the end of January we will discuss things with the two providers of their technology, and if there is a third system we will decide in February or March which will be used at the Confederations Cup, starting in June," said Christoph Schmidt of FIFA, who has just visited the Soccerex exhibition in Rio de Janeiro.
During Soccerex, Fifa secretary general Jerome Valcke defended the use of technology and explained that the system will send a signal to referee's stopwatches - with a one-second delay.
But Valcke says the referee will be the only one to see the information, and the ultimate decision will still fall to him.
He says it is a better system than stopping the action for several seconds or longer "to watch a video or to say if a goal has been scored through handball".