A relief pitcher (aka reliever, collectively the bullpen) is a pitcher who specializes is coming into a game started by another pitcher. The difference in usage patterns goes beyond when the pitchers are brought into the game. Unlike starters, who are given several days off after each appearance, relievers are expected to be able to pitch in several consecutive games.
A relatively recent development in relief pitching is the use of relievers in highly specific roles. Rather than using all relievers in essentially the same way, as teams do with their starters, managers now try to use each reliever in one of a small number of stereotypical roles that depend on the game situation and opposing batter. The most common roles include:
- Long relievers are brought into the game when the starting pitcher is pulled from the game early because of injury or ineffectiveness. The long reliever is expected to pitch until the point of the game where a starting pitcher normally would have been pulled, typically several innings. A long reliever who is used only in lost causes is called a mop-up man.
- Middle relievers are used later in the game than long relievers, typically in the 6th or 7th inning, and used for about one inning. Middle relievers are often brought into the middle of an inning when the starter has let several batters reach base. They may also be used in the late innings of games which their team is losing.
- LOOGYs are Lefty One Out GuYs, left handed relievers who are used to get one or two critical outs against the opponents' best left handed hitters. LOOGYs are almost always used with runners on base.
- Setup men are brought into the game in the 7th or 8th inning to bridge the gap between the starter or middle reliever and the closer. Setup men are normally reserved for close games.
- Closers are used to "close out" games that their team is winning. Most managers will reserve their closer for save situations, i.e. starting the 9th inning with a 1 to 3 run lead.
The latter three categories are collectively known as short relievers, i.e. relief pitchers not expected to pitch more than one or at most two innings in the normal course of a game.
Over the years a number of awards have been created to distinguish the most effective relief pitchers in the major leagues. First was the Reliever of the Year Award, created by The Sporting News in 1960. This was supplanted by the Rolaids Relief Award, introduced by Major League Baseball in 1976. It was discontinued after the 2012 season, and after a one-year hiatus, was replaced in 2014 by the Mariano Rivera Award in the American League and the Trevor Hoffman Award in the National League.
- Anthony Castrovince: "Eight-man bullpens gaining traction", mlb.com, February 25, 2015. 
- Alden Gonzalez: "Rethinking bullpen roles: Will teams buy in? Analytics have teams considering changes to traditional usage", mlb.com, March 16, 2016. 
- Gabe Lacques: "MLB’s bullpen revolution a hard sell for 162 games", USA Today Sports, February 21, 2017. 
- Tracy Ringolsby: "Royals exemplify evolved bullpen usage: Teams spreading workload over larger relief corps", mlb.com, February 8, 2016. 
- Fran Zimniuch: Fireman: The Evolution of the Closer in Baseball, Triumph Books, Chicago, IL, 2010.
- 2004 Article on Closers in The Hardball Times
- Article on LOOGYs in The Hardball Times, part 1
- Article on LOOGYs in The Hardball Times, part 2
- 2005 Article on Closers in The Hardball Times, part 1
- 2005 Article on Closers in The Hardball Times, part 2
- Article on the comparative difficulty of relief pitching versus starting in The Hardball Times