Interleague play was an innovation in Major League Baseball during the 1997 season. Prior to 1997, teams from the American League and National League would play only in spring training, the All Star Game, and during the World Series. During this time, the leagues had gained a unique identity and prided themselves on being a member of their league.
The idea for the leagues to play each other, began in 1933 when Bill Veeck Sr. suggested that teams play the other league for six weeks in the summer. The plan received some support but Veeck died later that year and his plan was shelved.
With the Washington Senators moving to Minnesota, there was talk of putting a National League team in New York, creating two nine-team leagues. With the imbalance, there would be one interleague series each day of the season. This plan is shelved when the American League expands by two teams in 1961 and the National League follows in 1962.
When the American League adopted the designated hitter in 1973, they also voted for interleague play but the plan was rejected by the National League. Commissioner Bowie Kuhn formed a committee to study the matter.
- First Game: San Francisco Giants at Texas Rangers, June 12, 1997
- First Pitch: Darren Oliver
- First Batter: Darryl Hamilton
- First Hit: Hamilton, led off the game with a single
- First Run: Damon Buford
- First Home Run: Stan Javier
- First Home Run by an American League Pitcher: Bobby Witt vs. Dodgers, June 30, 1997
Through the 2010 season, the American League held a 1,808-1,652 edge in interleague competition. It continued to dominate in the ensuing decade, as AL teams beat their NL rivals every season from 2004 to 2017. The senior circuit then reversed the trend in both 2018 and 2019, but at the start of 2020, the American League still held the edge, 3,166 to 2,898.
The following American League pitchers have homered in interleague play:
- Bobby Witt, 1997-06-30
- Dave Burba, 1998-06-07
- Dwight Gooden, 1999-06-11
- Esteban Yan, 2000-06-04
- Mark Hendrickson, 2003-06-21
- Jason Davis, 2004-06-20
- C.C. Sabathia, 2005-05-21
- Jason Johnson, 2005-06-08
- Zack Greinke, 2005-06-10
- Josh Beckett, 2006-05-20
- Kris Benson, 2006-06-17
- Jon Garland, 2006-06-18
- C.C. Sabathia, 2008-06-21
- Felix Hernandez, 2008-06-23
- Mark Buehrle, 2009-06-14
- Josh Beckett, 2009-06-14
- Zack Britton, 2011-07-03
- Nathan Karns, 2015-07-21
- Daniel Norris, 2015-08-19
- Anthony Ranaudo, 2016-07-28
No American League pitchers homered in interleague play in 2001, 2002, 2007, 2010 and from 2012-2014.
- Aria Gerson: "Happy birthday, interleague play! These are some of the best highlights from past 23 years", USA Today, June 12, 2020. 
- Gary Gillette and Pete Palmer: "Interleague Attendance Boost Mostly a Mirage", in The Baseball Research Journal, Society for American Baseball Research, Cleveland, OH, # 35 (2007), pp. 106-108.