What is Instant replay in Major League Baseball?

Major League Baseball uses instant replay review to allow league officials to review certain types of plays in order to determine the accuracy of the initial call of the umpires on the field. Reviews may be initiated either by a team's manager with limitations or by the umpires themselves. All instant replay reviews are examined by umpires at the Replay Command Center in New York City, who have the final decision as to whether to uphold or overturn the initial call.

MLB was the last of the four major North American professional sports leagues to implement an instant replay review system. Instant replay review was first implemented during the 2008 season. Under that system, only the umpire crew chief could initiate a review, and one or more members of the umpiring crew would review the video at the stadium and render the decision to uphold or overturn the call. Only boundary home run calls could be reviewed, either if the initial call was a home run but might not have been or if the initial call was not a home run but might have been.

The current instant replay system was implemented in the 2014 season. Under the current system, each manager is allotted one challenge per game, with additional challenges granted only if the previous one was successful. From the eighth inning on, the umpire crew chief is allowed to initiate his own replay review. The umpire crew chief is also allowed to initiate a review during any inning if the play in question is a boundary home run call.