Interleague play (MLB)

Interleague play was first introduced during the 1997 Major League Baseball season. Prior to that, matchups between AL teams and NL teams occurred only during spring training, the All-Star Game, other exhibition games (such as the Hall of Fame Game in Cooperstown, New York), and the World Series. Unlike modern interleague play, none of these contests, except for the World Series, counted toward official team or league records.

MLB's first regular-season interleague game took place on June 12, 1997, as the Texas Rangers hosted the San Francisco Giants at The Ballpark in Arlington. There were four interleague games on the schedule that night, but the other three were played on the West Coast, so the Giants–Rangers matchup started a few hours earlier than the others. Texas's Darren Oliver threw the game's first pitch and San Francisco outfielder Glenallen Hill was the first designated hitter used in a regular-season game by a National League team. San Francisco's Darryl Hamilton got the first base hit in interleague play, while Stan Javier hit the first home run, leading the Giants to a 4–3 victory over the Rangers.

From 1997 to 2001, teams played against the same division from the other league; for example, the American League West played teams from the National League West, typically scheduled to alternate between home and away in consecutive years. In 2002, however, the league began alternating which divisions played which divisions, and thus in 2002 the American League East played the National League West, the American League Central played the National League East, and the American League West played the National League Central. Matchups which had been of particular interest prior to this format — mainly geographic rivals — were preserved. Corresponding divisions were skipped once when this rotation began, but were put back in the rotation in 2006.

From 2002 to 2012, all interleague games were played prior to the All-Star Game (with the exception of games postponed by weather that were made up after the All-Star Game). Most games were played in June and early July, although beginning in 2005, interleague games were played during one weekend in mid-May.

The designated hitter (DH) rule was originally applied in the same manner as in the World Series (and the All-Star Game prior to 2010). In an American League ballpark, both teams had the option to use a DH, while in a National League ballpark, both teams' pitchers were required to bat. Teams from both leagues have both benefited and have been at a disadvantage when it comes to the DH rule in interleague play. For instance, Barry Bonds, who spent his entire career in the National League and actually won eight Gold Gloves earlier in his career, was used strictly as a DH later in his career when the San Francisco Giants played away interleague games due to his poor fielding. Conversely, Boston Red Sox slugger David Ortiz, who spent his entire career in the American League and was the Red Sox's regular DH, was assigned to play first base when the Red Sox had away interleague games, with the Sox choosing to give up good fielding in favor of retaining Ortiz's power hitting. With the introduction of the DH to the National League in 2020 and from 2022–present, this no longer applies.

In 2007, two teams – the Los Angeles Dodgers and the Baltimore Orioles – played six games with more than one interleague opponent. The Dodgers played both the Toronto Blue Jays and the Los Angeles Angels while the Orioles played both the Arizona Diamondbacks and the Washington Nationals. This happened again in 2012 as the New York Yankees played both the New York Mets and the Atlanta Braves for six games. The Miami Marlins also did this, playing both the Tampa Bay Rays and Boston Red Sox for six games each.

The first Civil Rights Game was an exhibition interleague game between the Cleveland Indians and St. Louis Cardinals at AutoZone Park in Memphis on March 31, 2007. The first regular season Civil Rights Game was an interleague game between the Chicago White Sox and Cincinnati Reds at Great American Ball Park on June 20, 2009.

Since the introduction of interleague play, two teams have shifted leagues: the Milwaukee Brewers from the American League to the National League in 1998, and the Houston Astros from the National League to the American League in 2013. As a result, a 2013 interleague series between the two teams made it the first time that two teams faced each other in an interleague series after both teams previously faced each other in an interleague series representing opposite leagues: the two teams met from September 1–3, 1997 (Houston in NL, Milwaukee in AL), then again from June 18–20, 2013 (Houston in AL, Milwaukee in NL). In both instances, the series took place in Houston, with the team representing the American League winning 2–1. From 1998 to 2012, both teams were division opponents in the National League Central.

For the 2020 season, all interleague games featured the DH, as the National League used the rule for the first time in its history due to health and safety measures related to the COVID-19 pandemic. The DH became permanent in the NL for the 2022 season.

• Baltimore Orioles vs Philadelphia Phillies: Played from 1997 to 2005. In 2006, the Washington Nationals replaced the Phillies as the Orioles' rival. Although the Nationals moved to Washington in 2005, the schedule for that season was already established, so the Washington-Baltimore rivalry series could not start until the next season. Therefore, in 2005 the Orioles played the Phillies as their rival for the last time, while Washington played Montreal's planned 2005 schedule, which included what would have been the Expos's interleague rivalry games with the Toronto Blue Jays.
• Texas Rangers vs Houston Astros (Lone Star Series): Played from 2001 to 2012. In 2013, the Astros moved to the American League West, and the two teams began play against each other 19 times a year as divisional rivals.
• Toronto Blue Jays vs Montreal Expos (Pearson Cup): Played from 1997 to 2004 (and by Washington against Toronto in 2005). By the time the Expos moved to Washington, D.C., to become the Washington Nationals for the 2005 season, the 2005 schedule already had been set, so the Nationals played what had been intended as Montreal's rivalry series with the Blue Jays that season. In 2006, the Nationals began playing the Baltimore Orioles annually as their interleague rival.

MLB

Crossover

Interleague play (NPB): Similar matchup in Japan.