What is baseball’s Automated Ball-Strike Challenge System (ABS)?
What is the Automated Ball-Strike Challenge System (ABS)?
How accurate are umpires in making ball and strike calls?
How does the ABS system work?
What changes does the ABS system bring to the strike zone?
The Automated Ball-Strike Challenge System (ABS) is an instant-replay mechanism that Major League Baseball (MLB) began using in the 2026 season and that allows players to challenge ball and strike calls made by home-plate umpires. Hawk-Eye cameras on the field track the baseball as it crosses the middle of home plate and determine whether challenges are successful.
There’s an old joke about an umpire who says, “It’s true that some are balls and some are strikes. But they ain’t nothing until I call ’em!” But in a few cases, at least, technology—not umpires—now has the final say.
About 94 percent of the time, which is pretty good. But that doesn’t mean that every baseball fan hasn’t screamed at an ump over a bad call.
In September 2025 Major League Baseball’s 11-person competition committee approved the ABS system, with six management representatives voting for it, although some of the players on the panel reportedly voted against it. Commissioner Rob Manfred praised the committee “for striking the right balance of preserving the integral role of the umpire in the game with the ability to correct a missed call in a high-leverage situation, all while preserving the pace and rhythm of the game.” Some in baseball decried the use of “robot umps.” But as Cleveland Guardians manager Stephen Vogt said at the time: “You can like it, dislike it, it doesn’t matter. It’s coming. It’s going to change the game. It’s going to change the game forever.”
How it works
- Each team gets two challenges per game, which they keep if the challenges are successful and the umpire’s call is overturned, incentivizing players to be highly selective about when to use them.
- Only a pitcher, catcher, or batter can challenge a ball or strike, and he does so by tapping his cap or helmet within about two seconds of the ump’s call.
- Everyone in the stadium and those watching at home see a replay of the pitch that shows whether the ball landed in the strike zone.
- For extra-inning games, teams that are out of challenges get one challenge at the start of each extra inning.
In going with ABS, professional baseball chose a middle ground between the status quo and having every ball and strike determined by technology, the so-called “robot umps” route. Manfred said that the “strong preference from players for the challenge format over using the technology to call every pitch was a key factor in determining the system we are announcing today.” UmpScorecards has calculated that MLB umpires get ball and strike calls correct about 94 percent of the time.
History
Minor baseball leagues began experimenting with ABS in 2019, when the independent Atlantic League used full ABS (as opposed to the challenge system) in its All-Star Game. That autumn MLB tried it out in the Arizona Fall League, which is a showcase of elite prospects. That was followed by using it at the Low-A Southeast League in 2021 and Triple-A in 2022. The challenge system debuted in the Florida State League in 2022. MLB tested both the challenge system and the full ABS system in Triple-A in 2023 and 2024.
In 2025 MLB took baby steps to using the ABS challenge at the major-league level, using it in nearly 300 games during spring training (where challenges were successful 52.2 percent of the time), then testing the system out at that year’s All-Star Game, where four of the five challenges were successful. The spring training experiment showed that ABS added less than a minute to each game.
This, of course, is not the first time professional baseball has introduced a system to allow an umpire’s call to be overturned. MLB first used instant replay in 2008, becoming the last of the four major North American leagues (the National Football League, National Hockey League, and National Basketball Association being the others) to adopt it. The initial use of instant replay was very limited: It was only used on home-run calls and only at the request of an umpire. But in 2014 MLB introduced a challenge system that allowed managers up to two challenges per game that could be used on most plays.
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Defining the strike zone
The Hawk-Eye system, invented in 1999, uses multiple cameras to track the trajectory of a ball. It was first used in cricket. It is now used in 25 sports worldwide, including:
- Tennis: for line calls
- Football (soccer): for goal-line calls
- Ice hockey: for replay reviews
- Football: for measuring first downs
- Baseball: for play challenges since 2014; ball and strike calls added in 2026
For ABS purposes the strike zone is 17 inches wide—not surprisingly, the same width as home plate. But things get a little more complicated with the height of the zone. Under this system, MLB takes measurements of every player’s height and defines the bottom of his zone as 27 percent of his height and the top as 53.5 percent of his height. That is slightly more hitter-friendly than the umpire-called strike zone, which sets the bottom at 24.2 percent of the player’s height and the top at 55.6 percent of the player’s height. On the flip side the ABS strike zone negates the batting crouch that players have long used to effectively shrink their strike zones.
So, for example, when comparing the towering Aaron Judge (6 feet 7 inches or 2.01 meters) and diminutive Jose Altuve (5 feet 6 inches or 1.68 meters), you might expect that Judge’s zone would be 13 inches bigger, given that he’s 13 inches taller. But even though Altuve uses a crouched stance to shrink his strike zone, under ABS—which doesn’t tailor the zone to a player’s crouch—the difference is only about four inches. This means that while Judge’s strike zone has shrunk with ABS, Altuve’s has grown.
Another expected change is that pitchers won’t get as many high strikes as they have in the past because hitters know they can lay off them now that ABS enforces a rigid strike zone. Also, the ABS strike zone is two-dimensional and measures where the ball crosses the plate at the halfway mark between the front and back of the plate. That means a ball that nicks a sliver of the plate at the front and then dives into the dirt will not be called a strike by ABS. Television broadcasts no longer use the technology that automatically indicates whether a pitch is a ball or a strike.
Fan reaction
At the start of 2026 spring training, MLB said in a presentation that a survey of 2025 spring training games found that 52 percent of fans said ABS had a “very positive” impact on the games, while 20 percent said it was “slightly positive.” Only 10 percent of fans said the system had a “slightly negative” or “very negative” impact.
