Details about each system are outlined below.
A camera-based system developed by the British company Hawkeye, which was bought last year by Japanese corporation Sony and already has systems used by tennis and cricket.
Six or seven high-speed cameras at both ends of the stadium, mounted on the roof, track the ball in flight and a computer system calculates exactly where the ball is on the pitch, sending an electronic message to a watch-like receiver worn by the match officials when it crosses the goal-line.
The only issue is whether the Hawk-Eye cameras would work in the very rare instance of the ball being completely covered by the goalkeeper's body.
The system is expected to be approved by IFAB but the cost of installing the system is likely to be relatively high.
FIFA have insisted that, initially at least, the pictures would not be allowed to be shown on television or on big screens in the stadium after any controversial incident, with only the officials being alerted whether the ball had crossed the line or not.
A joint Danish-German system, GoalRef uses magnetic fields to detect whether the ball has crossed the line.
Three magnetic strips are placed inside the outer lining of the ball, between the bladder and the outer casing, and when the ball crosses the line these are detected by sensors inside the goalposts and crossbar.
The sensors send out electronic waves which are disrupted when the ball crosses the line, and a computer then sends a message to the match officials' watch receivers in less than a second.
Installation costs should be lower than Hawk-Eye but still significant. There remains possible issues over deals with manufacturers to allow the magnetic strips inside their balls, but GoalRef have already been in contact with the manufacturers.