There are lots of advances being made in the technology of instant replay, however, the primary goal of instant replay has always been to review a video playback of what happened in the game.
There are many times during games that referees and umpires make mistakes that could end up affecting the score at the end of the game. Due to the importance of each and every call that referees and umpires make, it is vital that those calls are correct in order to keep the game fair for both teams and spectators.
The MLB was the last of the four major sports leagues to adopt instant replay as they did not until the 2008 season. They were hesitant to add instant replay as baseball purists did not want instant replay to break the tradition of putting the game in the hands of the umpires.
Originally, the only reviewable plays included determining fair or foul balls, the ball leaving the field of play, and if home runs are subject to fan interference. This replay system has expanded to include many more plays during games.
In the NBA, the first use of instant replay did not take place until the 2001-2002 season when it was used to review last second shots in games. Since it was first introduced, the NBA has expanded its use of instant replay to include more available reviewable plays.
The NFL began using instant replay in its most simple form during the 1986 season. The first use of it was used three plays into the game to determine a fumble recovery by Browns safety Al Gross in the endzone against the reigning Super Bowl Champion Bears.
The NFL has recently adopted a rule to be able to review pass interference penalties, however, this should not be allowed as it slows down the game and is just unnecessary as this season has shown that the challenges rarely ever work in the teams favor as the league likes to side with the original call made by the referees.
The NHL began using their instant replay in 1991 but did not begin using their current system until the 2003 season.
Instant replay in the NHL is used solely to determine goals on the ice. Coaches cannot challenge penalties or other plays that do not result in a goal.
All other replays that include penalties and hard hits that result in fines and suspensions do not come until the day after the game occurs. This keeps with the fast pace that hockey likes to maintain and discourages stoppages throughout the game as much as possible.
Coaches may use instant replay to challenge goals if they believe that an offside call was missed that led to the goal or a goalie interference occurred. However, even then coaches are discouraged from using challenges because if their challenge fails they must use the only timeout that they are given in the game.