'Robot umps'? See how MLB's new ABS challenge system works

Call them an extra set of eyes.

With the start of Major League Baseball's regular season this week, the Automated Ball-Strike (ABS) Challenge System makes its debut in all 30 ballparks. The system, made up of multiple high-resolution cameras, analyzes every pitch relative to the home plate umpire's strike zone.

While the technology was approved for this season by the league's Competition Committee last September, it has been game-tested since 2021 at the minor- and major-league level.

How does the ABS Challenge System work?

First, the technology does not exactly involve "robot umpires," as they have been nicknamed. Human umpires will still call every pitch from behind home plate as they always have. But ABS gives players a chance to contest a call in the moment.

Twelve high-speed Sony Hawk-Eye cameras placed at vantage points in each stadium triangulate the precise position of every pitch and assess customized strike zones based on each batter's height. The strike zone is a 17-inch-wide rectangle that will change slightly depending on the height of the batter. The top edge of the zone is 53.5% of the batter's height, and the bottom edge is 27% of the batter's height.

Only the three players closest to any given pitch − the batter, catcher and pitcher − are allowed to challenge a call. If any of those three disagree with a ball or strike called by the umpire, they can challenge the call by tapping their hat or helmet. The challenge must be made immediately − within two seconds of the pitch − and without any input from managers, the dugout or other fielders.

Fans in the stands can see the challenge result instantly. An animated pitch graphic appears on a video board showing the strike zone and the ball's relative position to it.

Challenges are quick, often resolved in under 15 seconds, according to MLB. In the 288 games played over spring training 2025, there were an average of 4.1 challenges a game, at an average 13.8 seconds a challenge.

If a call is overturned − a successful challenge − the challenging team retains its challenge. If the call stands, the challenge is unsuccessful and the team loses that challenge.

In the 2025 analysis, catchers were most successful with their challenges; they overturned 56% of plate calls, followed by batters at a 50% and pitchers at just 41%.

How many challenges do teams get?

At the start of every game, each team gets two challenges to a called ball or strike. The challenges can be used at any point in nine innings of gameplay.

If the game goes into extra innings, any team that is out of challenges at the end of the tied ninth inning will get one challenge for the 10th inning. Challenges do not accrue in extra innings − each team gets one challenge for additional innings if they've exhausted their challenges.

ABS has been used by Minor League Baseball's Single-A Florida State League since 2021 and all of Triple-A since the 2023 season. The technology made its MLB debut in spring training 2025.

SOURCE: Major League Baseball