PHOENIX — For more than 150 years, the game of baseball has relied on humans behind home plate to call balls and strikes.
This spring, for the first time in Major League history, players will be able to challenge those calls — with the assistance of an automated replay review system, similar to those used in tennis and soccer.
The "automated ball-strike challenge system," or ABS, as Major League Baseball calls it, will not be in place for the 2025 regular season. Its use during spring training is a test to see if there is a role for technology alongside the traditional human umpires in the big leagues.
In all, about 60% of spring training games will feature the ABS challenge system.
The earliest that ABS challenges could take effect in regular season MLB games would be 2026. League officials say they will not make a decision until they can analyze the results of this spring experiment and collect reactions from players, coaches and fans.
"Do they feel like baseball is a better game? Is it more fair?" said Morgan Sword, the MLB's executive vice president for baseball operations, the unit that leads research into rule changes. "This is a pretty big decision for the game of baseball that we want to make sure to get everybody to weigh in on."
Here's what to know:
How will it work?
In short, players will be able to challenge any call of a ball or strike. But there are limits: Only the pitcher, catcher or batter can call for a review, and they must do so immediately after the umpire announces the call, without any communication with the dugout.
Once a call has been challenged, an automated replay system will determine exactly where a pitch was located when it crossed home plate, using a set of cameras placed around the ballpark. A computer-animated replay will play on the scoreboard to announce the outcome of the challenge. The whole process takes about 15 or 20 seconds.
The automated strike zone will stay the same width for each player — 17 inches across, the width of home plate. Its height will change based on how tall the batter is. (The bottom of the zone will be set at 27% of a player's height and the top set at 53.5% of their height.)
Teams will start each game with two challenges. If a challenge is correct, meaning the system overturns an umpire's call, the team is allowed to keep that challenge to use again. But if a challenge was incorrect, and the umpire's call stands, the team loses that challenge.
In other words, once a team has made two unsuccessful challenges, they won't be able to challenge any more pitches for the rest of the game.
How will players use challenges?
For that we can look to the minor leagues. The ABS challenge system was in effect for many AAA games last season, along with Arizona Fall League games starting in 2023.
On average, there were about four challenges per game, the MLB says, roughly half of which were overturned. Challenges by the defense were slightly more successful on average than challenges by batters (54% to 48%).
Players also tended to save challenges for higher leverage situations, according to the league. Challenges were called almost twice as often in the ninth inning as they were early in the game. And more than 8% of pitches on a 3-2 count were challenged (compared to just 1.6% of pitches on a 0-0 count).
What are the players saying?
At spring training, there's been a lot of interest in the system from players and managers alike. With only two challenges available each game, teams are talking through how to strategize.
In the minor leagues, some teams decided that players shouldn't challenge too early in the count, or without runners on base. Others decided that only catchers should make challenges, not pitchers.
For players, calling a challenge can be nerve-wracking, as everyone in the stadium — players, coaches and fans — all turn to the scoreboard to see what the outcome will be.
"Your heart rate goes through the roof," said Colorado Rockies first baseman Michael Toglia, who played in AAA games last summer when the system was in place.
But when a challenge went his way, Toglio said, the satisfaction was tremendous — no more so than when a full-count strikeout was turned into a walk. "It changes your whole day. You sleep better," he said.
Why is MLB trying this out?
There's been enormous pressure on baseball to improve the accuracy of umpires. Missed calls can lead to lost tempers and ejections. Worse, they can lead to runs or strikeouts that change the outcome of games. In the social media era, with umpire "scorecards" and viral compilations of missed calls, fan outcry is louder than ever.
"There's a lot of pressure on umpires now to get these calls right, because there's such a spotlight," said Bob Melvin, the manager of the San Francisco Giants. "They're trying to take some of the controversy out of it."
Technology has already transformed baseball over the past few decades. And the pace of rule changes has sped up since Rob Manfred took over as MLB commissioner in 2015. Manfred's legacy already includes the pitch clock, introduced in 2023 to speed up games, and the adoption of the designated hitter in the National League in 2022.
"I think that everybody has come to appreciate that the changes in the game — it's not just about the minutes, it's about the action and the athleticism that the fans were looking for," Manfred said Tuesday. "And, like any institution, we're going to need to continue to change going forward."
Why not go all the way and have a fully automated strike zone?
In short, not very many people like that idea. The MLB says it conducted polls of fans, players and coaches, and a fully automated strike zone was consistently the least popular choice.
A vocal group of players and umpires "really did not like that system," said Sword, the MLB baseball operations executive. "[They] felt like it just was too big a change for baseball, that was unnecessary, [that] it was an overreach of technology that we didn't need in the game."
The objection comes from what people in baseball call "the human element" — in other words, the added dimensions to baseball that come from having a human behind home plate.
Pitchers and batters get a feel for the strike zone over the course of a game. Catchers frame pitches in an effort to influence the umpire's perception of a pitch. And catchers strike up relationships with umpires during the season — it can't hurt if it results in a friendly call here and there, the thinking goes.
"Whether or not [a relationship with the umpire] actually gets you strikes is hard to quantify," said Dan Wilson, the Seattle Mariners manager who played as a catcher in the major leagues from 1992 through 2005. "But it is a part of what you do back there, and it makes the game really fun."
With umpires, the "human element" used to be even more pronounced than it is now, said Colorado Rockies manager Bud Black, who was a pitcher for six major league teams in the 1980s and 1990s.
"The strike zone was, on a given night, at times, a little variable," Black said. "It didn't make it good or bad one way or the other. It's just what it was." Umpires sometimes gave generous calls to veterans, too, he added.
Technology has already changed that. In 2001, the league began using a system called QuesTec to evaluate umpire performances at some ballparks. In 2009, MLB expanded the evaluations to grade every umpire after every game.
"It's completely different now," said Melvin of the Giants, who caught for seven
teams during his MLB career. "They know they're being graded by a system, and they're just trying to get it right."
As MLB continues its ABS experiments, for some in the game, a big question remains: Is there a point at which the technology takes away more from baseball than it helps?
"I think that's probably true in life, right? It doesn't matter whether it's baseball or something else," Wilson said. "This is an area that's going to be hotly debated, I think, for at least a period of time."
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