When a game is tied after regulation play, sometimes seven innings in the minor leagues, but almost always nine innings in the major leagues, the game will go into extra innings.
In the traditional form of extra innings, the game continues as usual until the visiting team has scored more runs and the home team has completed a turn at bat, or the home team has scored more runs thus ending any chance for the away team to come back.
Before the advent of lights, extra inning games were often terminated and called a tie due to darkness. When lights were added to Wrigley Field in 1988, ties due to darkness were eliminated in the major leagues.
The most extra innings played in professional baseball is 24 by Rochester at Pawtucket in the International League on April 18, 1981. The record in the major leagues is 18 on May 1, 1920 between Brooklyn and Boston at Braves Field. The game was called a tie due to darkness.
In Nippon Pro Baseball, teams play three extra innings. If a team has not won after twelve innings the game is declared a tie. The exception is in the Japan Series and playoffs, where the game is played until it is decided conclusively. In the Korea Baseball Organization, the limit is also 3 extra innings, even in the Korean Series, resulting in occasional ties in the most important matches of the year.
In 2008, the International Baseball Federation introduced the concept of a tiebreaker rule for extra innings, having them begin with one or two runners already on base, depending on the variation of the rule being used, in order to speed up the conclusion of games in some international tournaments. This approach was strongly decried at first, but it was gradually adopted in other contexts, including the World Baseball Classic, international leagues and Minor League Baseball. Major League Baseball joined the movement in 2020 due to the shortened season caused by the global Coronavirus pandemic.
- Matt Kelly: "Celebrating 150 years of extra-inning baseball: Looking back at the best performances to happen after the ninth", mlb.com, June 14, 2020. 
- Philip J. Lowry: Baseball's Longest Games: A Comprehensive Worldwide Record Book, McFarland, Jefferson, NC, 2010.
- Joe Posnanski: "Extra innings can be too much of a good thing: Low Minors experimenting with beginning extra frames with runner on second", mlb.com, February 9, 2017.