Jared Shister, of the Chicago White Sox, delivers a pitch, while runner Luke Keaschall, of the Minnesota Twins, breaks for second on a stolen base attempt.
A Runner Tries to Steal
Once a runner reaches base, prior to the next pitch, they will take some manner of “leadoff”: a set of steps that basically gives them a head start running to the next base. And if a runner is feeling particularly emboldened, as the pitcher starts their delivery, they will start running to that next base. This is attempting to steal a base. The defense will attempt to tag the runner with the ball while they are not touching any base1; if they can do so, the runner is out. Otherwise, the runner has successfully stolen that base. Any base besides first base can be stolen, including home.
If there are multiple runners on base, they can attempt to steal bases at the same time (often called a “double steal”, especially when runners on first and second break for second and third at the same time). In some circumstances, you might also see a “delayed steal” where one runner breaks for a base, and another runner starts their own steal when the defense tries to throw the first runner out. These usually happen when there is a runner on first and third; the defense tries to get the out at second, while the runner on third breaks for home.
Stolen bases also make up part of an offensive strategy called the “hit and run,” where a baserunner breaks and the hitter will try to swing at the incoming pitch in the hopes of putting the ball into play. If it all works correctly, the hitter can get a hit, while the runner advances multiple bases, possibly even scoring. If it goes poorly, the result can be two outs; if the batter started with two strikes and swings through the pitch, they will have struck out, and if the defense then throws out the base-stealer, that’s another out.
Some base thefts can be chalked up to something called “defensive indifference.” This is a name for a play where the defense doesn’t really try to prevent the runner from advancing— usually because the runner represents a run that would not make a difference in the game outcome. For example, if the defensive teams is up by four runs and there is a runner on first, even if that runner scores, the defensive team will still be ahead. Late in the game, then, they may choose to focus on getting the batters out, rather than controlling the running game2.
If a runner attempts a steal, and the pitch is hit foul, the ball is declared dead; the runners have to reset back to where they were. Similarly on a hit by pitch, the ball is declared dead; the batter takes first, while the runners can keep their new bases as long as they are forced there. Otherwise, they also have to retreat.
Ultimately, while the results of a stolen base are binary (either the runner is safe or not), the result of the pitch can muddy the waters a bit. On a pitch that doesn’t result in a strikeout or a walk, then the count will advance accordingly while the base-stealing is resolved. If the pitch does result in a strikeout or walk, it can affect whether or not the stolen base still matters. Once a catcher attempts to throw out a runner who is stealing, thus putting the ball in play, there is the possibility of an error occurring, but... we'll get to that.