During his six seasons as a coach for the San Diego Padres, Rick Renteria frequently reviewed videotape of what he believed were umpires' questionable calls.

That experience led him to one unmistakable conclusion: Major League Baseball umpires are not always right.

But they usually are, far more often than not.

"It's been a very high rate of success on the umpires' side," Renteria says.

This season, Renteria's first as manager of the Chicago Cubs, will also be the first in which MLB teams can call on instant replay to challenge an umpire's call.

The technology will be used in selected spring-training games beginning this week, applied in a system similar to those already employed by the NFL and NHL. It basically works like this:

A manager can challenge one play during the first six innings of a game by verbally indicating his intention "in a timely manner." At that point, the play — just about anything other than a ball-strike call or a judgment ruling such as interference — will be reviewed from numerous camera angles by an umpire inside what MLB is calling its Replay Command Center in New York.

If the replay umpire finds "clear and convincing evidence to overturn the call," the challenge is successful and the manager is allowed one additional challenge — for a maximum of two per game. However, after the sixth inning the decision to use replay is left solely to the umpire crew chief on the field.

Nuances such as what defines "a verbal indication" and "a timely manner" have not been determined.

"There's going to be some bumps in the road, but that comes with any new initiative," Cleveland Indians General Manager Chris Antonetti says. "I don't think anyone's expecting it to be perfect. This is what we go out with initially, but everyone expects a year from now we will have learned some things and made some adjustments."

Baseball long resisted using video replay technology employed by nearly a dozen sports, from football and hockey to rodeo, cricket and rugby.

"Guys in baseball, we're a little bit stuck in our routine," Cleveland Manager Terry Francona says.

Baseball actually jumped ahead of other sports in 1999 when umpire Frank Pulli took a home run away from Florida Marlins slugger Cliff Floyd after looking at a television replay on a dugout monitor. It was the right call, but Pulli and his crew were chastised by baseball executives.

"Use of the video replay is not an acceptable practice," Leonard Coleman, then president of the National League, said in a statement at the time. "The integrity of the game requires that judgments be left to on-field personnel."

Turning a blind eye didn't work long-term, though, because fans watching at home with the benefit of instant replay soon grew tired of blown calls occasionally affecting important games. The first chip at tradition came in August 2008, when Commissioner Bud Selig approved the limited use of replay for boundary calls involving home runs.

Some umpires were initially opposed to any use of video replay, but the umpires union agreed to expanding the rule this season with far more rank-and-file support than might have been expected.

"It's going to help us," says Alfonso Marquez, who has been a big league umpire for 15 seasons. "Everybody knows technology is here to stay and it's just going to get better. The human eye is never going to be able to keep up."

Marquez said it's a terrible feeling when an umpire or a member of his crew makes a call that he later learns from video was wrong. "We just want to get it right," he says.

As spring-training games begin, there are more questions than answers about exactly how the process will work.

"There's going to be a lot of wait-and-see-type of things. How the video's set up, what kind of time you have to get confirmation or whatever," Oakland Manager Bob Melvin says. "So we're not really sure how it's going to play out."