In Major League Baseball, the American League Championship Series (ALCS), played in October, is a round in the postseason that determines the winner of the American League pennant. The winner of the series advances to play the winner of the National League Championship Series in baseball's championship, the World Series.

It started in 1969, when the American League was reorganized into two divisions, East and West. The winners of each division played each other in a best-of-five series to determine who would advance to the World Series. In 1985, the format changed to best-of-seven. In 1994, the league was restructured into three divisions, with the three division winners and a wild-card team advancing to a best-of-five postseason round, known as the American League Division Series (ALDS). The winners of that round then advanced to the best-of-seven ALCS. This is the system currently in use.

The ALCS and NLCS, since the expansion to best-of-seven, are always played in a 2–3–2 format: Games 1, 2, 6, and 7 are played in the stadium of the team that has home field advantage, and Games 3, 4, and 5 are played in the stadium of the team that does not. The series concludes when one team records its fourth win. Since 1995, home field advantage has been given to the team that has the better regular season record, unless that team happens to be the Wild Card team. In that case, the other team gets home field advantage, because by rule the Wild Card team is never allowed home field advantage in a Division Series or LCS. In the event that both teams have identical records in the regular season, home field advantage goes to the team that has the winning head-to-head record. From 1969-1993, home field advantage alternated between the two divisions.

Mariano Rivera (born November 29, 1969) is a Panamanian right-handed baseball pitcher who has played 18 seasons in Major League Baseball (MLB) for the New York Yankees. Nicknamed "Mo", Rivera has served as a relief pitcher for most of his career, and since 1997, he has been the Yankees' closer. A 12-time All-Star and five-time World Series champion, he is MLB's all-time leader in saves (608) and games finished (892). His accolades include five American League (AL) Rolaids Relief Man Awards, the 1999 World Series Most Valuable Player (MVP) Award, and the 2003 AL Championship Series MVP Award.

Rivera was signed by the Yankees organization in 1990 as an amateur free agent in Panama, and he debuted in the major leagues in 1995. Initially a starting pitcher, he struggled in the role and was consequently converted to a relief pitcher. After a breakthrough season in 1996 as a setup man, he became the Yankees' closer in 1997. In the following years, he established himself as one of baseball's top relievers, leading the major leagues in saves in 1999, 2001, and 2004. Rivera's presence in the late innings of games to record the final outs has contributed to the Yankees' success in the late-1990s and 2000s, particularly in the postseason where he has set numerous records, including lowest earned run average (ERA) (0.70) and most saves (42). His pitching repertoire consists primarily of one pitch, a sharp-breaking, mid-90s mile per hour (mph) cut fastball that has been called an all-time great pitch.

David Américo Ortiz Arias (born November 18, 1975), nicknamed "Big Papi", is a Dominican-American professional baseball player with the Boston Red Sox of Major League Baseball. Previously, Ortiz played with the Minnesota Twins from 1997 until 2002. Ortiz is a seven-time All-Star and holds the Red Sox single season record for home runs in a regular season with 54, set during the 2006 season. In 2005, Red Sox ownership presented him with a plaque proclaiming Ortiz "the greatest clutch-hitter in the history of the Boston Red Sox".

Ortiz graduated from Estudia Espallat High School in the Dominican Republic and in 1992 he was signed by the Seattle Mariners who listed him as "David Arias". He played for the Wisconsin Timber Rattlers, a Mariners farm team, until 1996, when he was traded to the Minnesota Twins as the player to be named later in an earlier trade for Dave Hollins. When he arrived in Minnesota, he informed the team that he preferred to be listed as "David Ortiz."

He made his Twins debut in September 1997. For a few years, he was moved back and forth between the Twins and their minor league affiliate in Connecticut, the New Britain Rock Cats. The Twins advanced to the American League Championship Series that year, where they lost to the Anaheim Angels. Despite showing flashes of talent, Ortiz's time with the Twins will be remembered as a series of injuries and inconsistency both in the field and at the plate. Ortiz suffered wrist injuries in both 1998 and 2001. He continued to experience knee problems in early 2002 that plagued him throughout the season, despite hitting 32 doubles, 20 home runs and 75 RBIs in 125 games. He was released by the Twins after the season. In six seasons with the Twins, Ortiz hit 58 home runs and 238 RBI.

Bronson Anthony Arroyo (born February 24, 1977 in Key West, Florida) is a Major League Baseball right-handed pitcher for the Cincinnati Reds and an aspiring musician. He has previously played for the Pittsburgh Pirates between 2000 and 2002 and Boston Red Sox from 2003 to 2005. Bronson is a Cuban-American and grew up in Brooksville, Florida and pitched for Hernando High School, graduating in 1995.^{[citation needed]}

Signed by the Pittsburgh Pirates in the third round of the 1995 draft, Arroyo made his major league debut with Pittsburgh in 2000.

He came to the Boston Red Sox after being selected off waivers before the 2003 season.

On August 10, 2003, Arroyo pitched the fourth nine-inning perfect game in the 121-year history of the International League as the PawSox beat the Buffalo Bisons 7–0 at McCoy Stadium. He needed 101 pitches to throw his masterpiece (73 strikes), struck out nine, and got 10 fly outs and eight ground balls from the Buffalo 27 batters. He went to a three-ball count to just three hitters all game.