American Major League Baseball (MLB) started using instant replay in August of 2008 to review specific types of calls. Before that, instant replay in baseball had only been used on an irregular basis, usually by individual referees who had doubts about their decision. As of 2008, instant replay can be used to verify three types of calls: whether home run balls were fair or foul, whether the a home run ball actually went out of the ball field or not, and whether or not a spectator interfered with the progress of a ball hit in a home run. These are all known as "boundary calls" since they have to do with whether a hit is considered a home run or not.
- To determine if the ball is fair or foul: if the ball is fair, the home run stands, if it is foul, then it doesn't count.
- To determine if the ball actually left the playing field or not. There are three possible outcomes here: if the ball clears the wall, it can be a home run; if it stays in the field it can be considered "live" and is still in play; and if it leaves the field entirely it can be considered a home run.
- To determine if a spectator interfered with a live ball by touching it or getting in the way of a player trying to catch it. This is only counted as interference if the ball is still in the field; once it passes over the wall spectators can try to catch it even if they get in the way of a player.
Instant replay had been used on a sporadic basis since the 1960s, when baseball Commissioner Bowie Kuhn reviewed video of a controversial call in the 1969 World Series. It was used on and off again until 1999, when referees were actually reprimanded by the American League Office for looking at footage of a ball that hit the scoreboard in a game between the Florida Marlins and the St. Louis Cardinals. The Marlins protested, saying that watching the footage set a bad precedent, and the use of instant replay in baseball was banned until the MLB officially sanctioned it in 2008. It was officially used for the first time in the World Series the next year when Yankees player Alex Rodriguez hit a ball that hit a camera.