Hawk-Eye is the electronic system famously used in tennis to review line calls.
Developed in the United Kingdom by Paul Hawkins, Hawk-Eye began as a cricket television-replay system in 2000. It uses images from multiple high-speed cameras, triangulating the ball’s position to reconstruct its flight and estimate where it landed. In tennis, that result is displayed as a visual replay often called Shot Spot.
The technology entered elite tennis in the mid-2000s. The 2006 US Open became the first Grand Slam tournament to use Hawk-Eye during play for player challenges, while the 2006 Hopman Cup was the first elite event to let players challenge point-ending calls using the system. It was initially a review aid rather than a fully automatic live line judge.
A common mix-up is Cyclops, an older electronic system used mainly for service calls. ShotLink is associated with golf, and GoalControl is a football goal-line system. Modern Hawk-Eye Live can make real-time in-or-out calls, although some tournaments and surfaces have used other technologies or traditional inspection of clay-court marks.