A relief pitcher or reliever is a baseball or softball pitcher who enters the game after the starting pitcher is removed due to injury, ineffectiveness, fatigue, ejection, or for other strategic reasons, such as being substituted by a pinch hitter. Relief pitchers are further divided informally into closers, set-up relief pitchers, middle relief pitchers, left-handed specialists, long relievers, and mop-up relief pitchers.
The importance of relief pitchers has increased significantly since the 1960s. This change in mindsets can be seen by watching any major game, as well as by monitoring the salaries of such pitchers. In decades past, the relief pitcher was merely an ex-starter who came into a game upon the injury, ineffectiveness, or fatigue of the starting pitcher. The bullpen was for old starters who had lost the ability to throw effectively. Many of these pitchers would be able to flourish in this diminished role. Those such as Dennis Eckersley, as with many others, actually prolonged their diminishing careers and often sparked them to new life. The added rest to their arms as well as the lessened exposure of their abilities became an advantage many would learn to capitalize on. Facing some batters only once a season, the opposition would have greater difficulty in preparation for the game.
Recently, the relief pitcher's position has become more of a career, rather than a degradation position. Many of today's top prospects are those of relief pitchers. Furthermore, the game today often calls for those in the bullpen to step up and become occasional starters. Their limited exposure, as was stated before, often can give these chance starters an edge. This ability to fill starts effectively often leads to less down time of the main starters, as well as longer, healthier careers. For example, the lengthy careers of such pitchers as Randy Johnson and Curt Schilling are examples of pitchers whose careers were extended by the assistance of relief pitchers. Furthermore, many pitchers who started as relief pitchers have gone on to prosper and remain starters, like Pedro Martínez.
Another reason for the advancement of the skill in the relief positions is due to the game's ever expanding need for a psychological edge. A batter who can't hit well off the movement of a left-hander's curveball will often be pitched to by a left-hander in a late inning situation. In late innings such as the 7th, 8th, and 9th, many managers are known for changing pitchers often as the situation dictates, notably Tony La Russa and Tom Lasorda
As of October 2010, there are five pitchers currently in the Baseball Hall of Fame chiefly for their accomplishments as relief pitchers: Goose Gossage, Hoyt Wilhelm, Rollie Fingers, Bruce Sutter, and Dennis Eckersley (though Eckersley and Wilhelm had significant careers as starting pitchers).