Major League Baseball (MLB) implemented the current instant replay review system in 2014. It was an expansion on a previous system that was implemented in 2008 and only applied to home run plays. In 2014, the MLB created a crew of umpires who would be stationed in New York and be in charge of deciding the outcomes of reviewed plays. The system also expanded to include other plays besides just disputed home runs, and gave managers the ability to request an instant replay challenge (prior to 2014, only umpire crew chiefs could officially request a replay review).
Starting in 2014, the instant replay review system also expanded to include plays besides just disputed home runs. Some of the plays that were newly eligible for instant replay review in 2014 include: force plays, tag plays, fair/foul ball calls (applies to balls hit into the outfield ONLY), whether a baserunner skipped a base, whether a batter was hit by a pitch, whether a baserunner scored before or after the third out was made (also called time plays), fan interference, errors in the ball-strike count, and more. Managers are allowed one challenge at the start of the game; if they challenge a call and it gets overturned (meaning the replay officials decided that the original call was incorrect), managers would retain their challenge, meaning they would be able to make one more challenge, at their discretion, later in the game. If they challenge a call and it stands, they lose their challenging abilities for the rest of the game. The rules of the previous 2008 system were kept, meaning the umpire crew chief (most experienced umpire) could request a replay review of a home run play at any time during the game.
More changes were made to the instant replay review system in 2015. Previously, managers had to leave the dugout and approach the umpire to request an instant replay; starting in 2015, they could make the request from the dugout. Additionally, managers now retained their challenge after every overturned call. (Previously, they could only retain their challenge after the first overturned call.) Managers also were able to start postseason games and tie-breaker games with two challenges, instead of just one like in regular season games.
In an effort to speed up the game, MLB implemented time limits on various aspects of the sport, this included changes to the instant replay review system. Managers now had to request a replay within thirty seconds after the play. Previously, they had up until the start of the next play, and would sometimes try to stall the game between possibly-disputed plays to have more time to make a decision. Also, the replay crew in New York now had a two-minute time limit to reach a final conclusion. Another change was that the umpire crew chief could now, at his discretion, request a replay review of any eligible play, not just home run plays, starting in the eighth inning (the previous rule also allowed this, but starting in the seventh inning).