Rule 8.04 states that:
When the bases are unoccupied, the pitcher shall deliver the ball to the batter within 12 seconds after he receives the ball. Each time the pitcher delays the game by violating this rule, the umpire shall call "Ball." The 12-second timing starts when the pitcher is in possession of the ball and the batter is in the box, alert to the pitcher. The timing stops when the pitcher releases the ball.
However, there is no similar rule that covers a situation when there is one or more baserunners on base, and traditionally, the existing rule was rarely enforced in the major leagues as umpires did not keep a stopwatch trained on the pitcher.
Major League Baseball began to tinker with the idea of a pitch clock during the 2014 Arizona Fall League season, in an attempt to speed up the game. Following that successful experiment, owners decided at their meeting of January 15, 2015 to introduce a pitch clock in the minor leagues at the AA and AAA levels beginning that season. In this case, the 12-second limit remains in place for situations in which there is no runner on base, and a 20-second limit is in effect for situations when there is one or more baserunners.
The intention was to eventually extend the use of the pitch clock to the major league level. It is modeled to some extent on the "shot clock" which exists in basketball and whose purpose is also to speed up the game. On January 18, 2018 reports emerged that Commissioner Rob Manfred was prepared to unilaterally impose a 20-second pitch clock with no runners on base that season. A first violation would result in a warning by the umpire, and any subsequent one by a ball being added to the count. However, the Commissioner was unable to come to terms with the Players Association on the implementation of the policy. The two sides agreed to other measures to speed up the pace of play, including limits on mound visits and a reduction of the time allotted between innings. In 2019, the clock was introduced in spring training games as a practical test with a view towards implementation during that or a future regular season. That year, a 20-second pitch clock was also put in effect for international games, starting with the 2019 Pan American Games. In 2021, baseball went even further by lowering the time between pitches to 15 seconds in some Low A minor leagues, to test how this would work.
- David Adler: "MLB could implement pitch clock this season", mlb.com, January 19, 2018. 
- Bob Nightengale: "Rob Manfred, MLB considering radical changes to game", USA Today Sports, August 18, 2016. 
- Bob Nightengale: "Like it or not, MLB to implement pitch clock in 2018", USA Today Sports, July 11, 2017. 
- Bob Nightengale: "Pitch limit or not, MLB-union hostilities already on the clock", USA Today Sports, January 18, 2018.