Umpires are responsible for enforcing on-field rules and rendering decisions on judgment calls such as: Whether a batter or baserunner is safe or out, and whether a pitched baseball is a strike or a ball.
A regular-season contest will have four umpires: one behind home plate and one stationed near each of the other three bases. Each umpire makes "out" or "safe" decisions at his designated base, and the home-plate umpire is responsible for calling balls and strikes on each pitch that is thrown. The first-base umpire determines whether batted balls were hit into fair or foul territory down the right-field line past the first-base bag, with the third-base umpire doing the same on balls hit down the left-field line beyond the third-base bag. Umpires at first and third base can also assist the home-plate umpire in determining whether a batter offered at a pitch or "checked his swing." If the latter is determined, a pitch outside the strike zone is recorded as a ball.
Umpires also have the jurisdiction to eject players, coaches or managers who break certain rules or do not display proper on-field conduct.
Unlike the umpires stationed in the field, the home-plate umpire stands behind the plate and wears a facemask and a heavy chest pad in order to protect against batted balls that are fouled directly backward.
During postseason contests, Major League Baseball adds left-field and right-field umpires to help ensure accurate calls on baseballs hit down the fair/foul lines. Umpires stationed in the outfield can also assess if a batted ball hit in their vicinity was caught on the fly or trapped in a fielder's glove after bouncing on the grass.
Major League Baseball also employs replay officials -- full-time Major League umpires who work shifts in the Replay Command Center in New York. Replay officials review all calls subject to replay review and decide whether to change the call on the field, confirm the call on the field or let stand the call on the field due to the lack of clear and convincing evidence. Replay officials also work as on-field umpires during the season.