In baseball, the bullpen (or simply the pen) is the area where relief pitchers warm up before entering a game. A team's roster of relief pitchers is also metonymically referred to as "the bullpen". These pitchers usually wait in the bullpen if they have not yet played in a game, rather than in the dugout with the rest of the team. The starting pitcher also makes their final pregame warm-up throws in the bullpen. Managers can call coaches in the bullpen on an in-house telephone from the dugout to tell a certain pitcher to begin their warm-up tosses.
Each team generally has its own bullpen consisting of two pitching rubbers and plates at regulation distance from each other. In most Major League Baseball parks, the bullpens are situated out-of-play behind the outfield fence.
The term first appeared in wide use shortly after the turn of the 20th century, and has been used since in roughly its present meaning. According to the Oxford English Dictionary the earliest recorded use of "bullpen" in baseball is in a Cincinnati Enquirer article published on May 7, 1877, in which writer O.P. Caylor noted in a game recap:
"The bull-pen at the Cincinnati grounds with its 'three for a quarter crowd' has lost its usefulness. The bleacher boards just north of the old pavilion now holds the cheap crowd, which comes in at the end of the first inning on a discount."
In October 2021, the People for the Ethical Treatment of Animals (PETA) suggested that Major League Baseball change the term to "arm barn," viewing bullpen as "outdated". A press release from PETA said: "Words matter, and baseball ‘bullpens’ devalue talented players and mock the misery of sensitive animals". As of 2023[update], no such change has been made.
In most major league stadiums, the bullpens are located out of play just behind the outfield fences. Commonly, the bullpens are separated from each other, and each team's is located on the side of the field corresponding with the same team's dugout. However, there are exceptions. In a few ballparks, the team's bullpens are opposite their own dugouts, which allows the manager to more easily watch the pitchers warming up from his dugout. A recent trend is the installation of mesh outfield walls in front of the bullpen to allow the bullpen to be more easily seen by both fans and the manager in the dugout, as well as to allow the players in the bullpen to more easily see what is occurring on the field.
Certain ballparks have their outfield bullpens in unusual configurations. Petco Park features the home bullpen behind the outfield fence and the visitor's bullpen behind that and one level higher. The visitors' bullpen was moved to that location from foul territory after the 2012 season.
As of the 2022 season, Oakland Coliseum and Tropicana Field are the only major league ballparks whose bullpens are located in foul territory.
Between 1950 and 1995, varying numbers of MLB teams used vehicles to transport pitchers from the bullpen to the mound. These bullpen cars ranged from golf carts to full-sized cars. The 1950 Cleveland Indians were the first to use a bullpen car. The last use of a bullpen vehicle in this time was a motorcycle and sidecar used by the Milwaukee Brewers in 1995. However, the Arizona Diamondbacks, Detroit Tigers, and the Washington Nationals have since given relief pitchers the option of using a bullpen cart in the 2018 season.