In baseball, the pitcher is the player who throws the baseball from the pitcher's mound toward the catcher to begin each play, with the goal of retiring a batter who attempts to either make contact with it or draw a walk. In the numbering system used to record defensive plays, the pitcher is assigned the number 1. In the National League and the Japanese Central League, the pitcher also bats. Starting in 1973 with the American League and spreading throughout the 1980s and 1990s, the hitting duties of the pitcher have generally been given over to the position of designated hitter, a cause of some controversy.

In most cases, the object of a pitch is to deliver the ball to the catcher without allowing the batter to hit the ball. The ball is delivered in such a way that the batter either can't hit a pitch through the strike zone, hits the ball poorly (resulting in a pop fly or ground out), or is fooled into swinging at a pitch outside of the strike zone. If the batter elects not to swing at the pitch, it is called a strike if the ball passes through the strike zone and a ball is when a pitch doesn't pass through the strike zone and the batter doesn't swing. The batter can also check swing. A check swing is when the batter swings half way when the batter thinks the pitch will be a ball. If the batter successfully check swings and the pitch is out of the strike zone, it is called a ball.

There are two legal pitching positions, the windup and the set (also called the stretch). Either position may be used at any time; typically, the windup is used when the bases are empty and the set is used when runner(s) are on base. Each position has certain procedures that must be followed. A power pitcher is a pitcher who relies on the velocity of his pitches to succeed. Generally, power pitchers record a high percentage of strikeouts. A control pitcher succeeds by throwing accurate pitches and thus records few walks.

Nearly all action during a game is centered around the pitcher for the defensive team. A pitcher's particular style and skill heavily influences the dynamics of the game and will often determine the victor.

The type and sequence of pitches chosen depends upon the particular situation in a game. Because pitchers and catchers must coordinate each pitch, a system of hand signals are used by the catcher to communicate choices to the pitcher, to which the pitcher either vetoes or accepts. The relationship between pitcher and catcher is so important, that some teams use more than one starting catcher; selecting the catcher for a particular game based on who the starting pitcher is. Together, the pitcher and catcher are known as the battery.