Robot umpires are set to enter the major leagues in 2026 following Major League Baseball’s 11-man competition committee’s decision on Tuesday to approve the use of the Automated Ball/Strike System.

ABS will be implemented as a challenge system where the human umpire makes the initial call, which can be contested and reviewed by the computer. Robot umpires have been trialed in the minor leagues since 2019, with more recent testing at Triple-A starting in 2022, during MLB spring training this year, and at the summer’s All-Star Game in Atlanta.

Here’s what to know about MLB’s robot umps.

How does the Automated Ball-Strike System work?

Stadiums are equipped with cameras that monitor each pitch to determine if it crossed the plate within the strike zone. In early tests, umpires wore earbuds to hear “ball” or “strike” and communicated the call to players and fans using traditional hand signals.

The challenge system introduces a new aspect. Human umpires make the call for every pitch, but each team can challenge two calls per game. Teams that deplete their challenges receive one additional challenge per extra inning. Challenges are retained if successful, paralleling the regulations for MLB teams using video reviews, which initially focused on home run calls in August 2008 and were extended to various calls for the 2014 season.

Challenges can only be made by the batter, pitcher, or catcher through a tap on their helmet or cap, without help from the dugout. A challenge must be made within 2 seconds, and the pitch and strike zone graphic is shown on the scoreboard and broadcast feed. The umpire then updates the count.

MLB estimates the process averages 17 seconds.

What is the technology?

A Hawk-Eye pose-tracking system monitors pitches and determines if they are within a strike zone tailored to the height of each batter, who is measured without shoes before a team’s first test game. MLB estimates the calibration process takes less than one minute per player.

While the strike zone actually called by big league umpires tends to be oval in shape, the ABS strike zone is a rectangle, as in the rule book.

Developing a consensus on what a computer strike zone should be has been an issue.

So what is the MLB strike zone?

MLB has changed the shape of the ABS strike zone several times.

It started with a 19-inch width in 2022, then dropped it to 17 inches – matching the width of home plate. Narrowing the strike zone led to an increase in walks and only small changes in strikeout rates.

The top of the strike zone was 51% of a batter’s height in 2022 and 2023, then raised to 53.5% in 2024 after pitchers complained. The bottom of the strike zone has been 27% since 2022 after initially being set at 28%. A batter’s stance is not taken into account.

ABS makes the ball/strike decision at the midpoint of the plate, 8 1/2 inches from the front and 8 1/2 inches from the back. The contrasts with the rule book zone called by umpires, which says the zone is a cube and a strike is a pitch that crosses any part. Big league umpires call roughly 94% of pitches correctly, according to UmpScorecards.

Where has ABS been tested?

ABS, which utilizes Hawk-Eye cameras, has been tested in the minor leagues since 2019. The independent Atlantic League trialed the system at its 2019 All-Star Game and MLB installed the technology for that’s year Arizona Fall League of top prospects. The ABS was tried at eight of nine ballparks of the Low-A Southeast League in 2021, then moved up to Triple-A in 2022.

At Triple-A at the start of the 2023 season, half the games used the robots for ball/strike calls and half had a human making decisions subject to appeals by teams to the ABS.

MLB switched Triple-A to an all-challenge system on June 26, 2024, then used the challenge system this year at 13 spring training ballparks hosting 19 teams for a total of 288 exhibition games. Teams won 52.2% of their ball/strike challenges (617 of 1,182) challenges.

At this year’s MLB All-Star Game, four of five challenges of plate umpire Dan Iassogna’s calls were successful.

How successful are teams at challenging?

Success rate have hovered around 50% in the minors. At Triple-A this season, the success rate dropped to 49.5% from 50.6%. Defenses – usually catchers – have been more successful, winning 53.7% of challenges compared to 45% by batters. Challenges increased to 4.2 from 3.9 per game through Sunday.

in 2024 at Triple-A, just 1.6% of first pitches were challenges, but the figure increased to 3.9% for two-strike pitches, 5.2% for three-ball pitches and 8.2% for full counts.

Challenge percentages were higher later in the game. While 1.9% of pitches were challenged in the first three innings, 2.5% were challenged from the fourth through the sixth, 2.8% in the seventh and eighth and 3.6% in the ninth.