Instant replay
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 (e.g., spectator interference or a foul ball near the foul pole) or if the initial call was not a home run but might have been (e.g., the ball hit an object such as a railing beyond the outfield wall and then bounced back onto the field).