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.
The role of relief pitchers has evolved over the years. Originally, it was not a glamorous one, as it was reserved for younger pitchers getting their feet wet in the majors, or older ones on the way out. The understanding was that all the better pitchers were used as starters. Managers would often use one of their starters to close out important games. There were some exceptions, pitchers who would spend most of their careers as relievers and be effective in the role - Firpo Marberry and Ace Adams are examples - but the recognition of a "relief ace" did not come until the late 1940s and early 1950s, when the New York Yankees were very successful with Johnny Murphy and Joe Page in the role, and Jim Konstanty led the 1950 Philadelphia Phillies to a pennant by pitching out of the bullpen. The role of the relief pitcher became more defined after that, but it was still rare for teams to train young pitchers to become relievers. All the best pitching prospects in the minor leagues were used as starters until the 1990s, and it was standard for scouts to proclaim that teams should steer clear of anyone who was used primarily as a reliever (or designated hitter) in college, as these roles were for "incomplete" players and not real prospects.
The stigma associated with relief pitchers has largely disappeared at the major league level, as it has become understood that certain pitchers are more suited to pitching in relief and can provide extremely valuable contributions in the role. For example, middle relievers were until the 21st century largely excluded from consideration for the All-Star Game, but this has now changed. In parallel relievers have begun to represent an ever-larger segment of a major league roster, with most teams now having seven or eight relievers on their roster in permanence. However, some of the negative perception associated with the role still persists in the minor leagues, where closers are valued, but other members of the bullpen not as much. They tend to be considered as non-prospects or "organization soldiers", whose ceiling is limited to making a few short appearances on a major league roster to help out an otherwise tired bullpen.
- Anthony Castrovince: "Eight-man bullpens gaining traction", mlb.com, February 25, 2015. 
- Ray Glier: "Ups and downs of minor league bullpens", USA Today Sports, July 5, 2018. 
- 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. 
- Pete Palmer: "Relief Pitching Strategy: Past, Present, and Future?", Baseball Research Journal, SABR, Vol. 47, Nr. 1 (spring 2018), pp. 45-52.
- 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