The strike zone in baseball is a tricky subject as the strike zone changes with each batter. Accordingly, there is no "automatic" strike zone and much of what is a strike or not a strike is left to the judgment of the home plate umpire. In short, the strike zone extends from the batter's knees to the letters across the front of his uniform. To be called a strike, the pitcher must throw the baseball across home plate, an area 17 inches wide (0.4318 meters), and between the knees and letters as indicated.

In professional baseball, a player who stands 5' 6" tall (1.67 meters), for example, will have a much smaller strikes zone that will a 6' 4" (1.93 meters) homerun hitter. Moreover, the focus is not on where the ball is caught by the catcher, but rather where the ball is as it passes the front of home plate.

The home plate umpire stands directly behind the catcher and he is there largely to call balls and strikes. If a thrown ball is outside of the strike zone, regardless of whether it is high, low, inside, or outside, the umpire will call the pitch a "ball." If the pitch is within the strike zone, the umpire will call a "strike." When a pitcher throws three strikes to the same batter, then the batter will be called "out" by the umpire. However, if the pitcher tosses four balls before throwing three strikes or before the batter hits the ball into play, the batter will be awarded first base. This is known as a walk.

A disciplined batter will only swing at pitches that are in the strike zone. However, even the best hitters in the game are fooled by pitchers who throw pitches that look as if they are in the strike zone, only to drop, dart, or curve out of the zone just before reaching home plate. When a batter swings at such a pitch, even if it is out of the strike zone, it is still a strike.

Nolan Ryan, a flame-throwing right hander who pitched for such teams as the Mets, Angels, Astros, and Rangers, is the career leader in strikeouts. Over the course of his 27-year major league career, Ryan struck out 5,714 batters. Pitchers Steve Carlton, Roger Clemens, Randy Johnson, and Bert Blyleven round out the top five strikeout pitchers in major league history.

No two umpires will have the exact same strike zone. Some umpires tend to have a wider strike zone, calling strikes on pitches just outside of the plate, and some umps have been known to "squeeze" the strike zone, meaning they don't call many pitches on the corners for strikes. To help gain more uniformity amongst umpires, major league baseball has installed a tracking system designed by the company, Questec, which monitors the home plate umpires calls during the course of the game. The Questec tracking device is used in 10 major league ballparks and after the game, a CD ROM disc is prepared and used to evaluate the umpire's performance