A baserunner or runner is a player for the batting team who has safely reached one of the three bases. The objective of a baserunner is to advance to the next base, until he reaches home plate and scores a run. Conversely, the objective of the fielders is to prevent opposing baserunners from advancing or scoring.
Becoming a baserunner
A batter becomes a baserunner when he reaches first base safely. This can be achieved through several different means:
- by hitting the ball for a base hit;
- by reaching first base without ability to be put out as a result of a base on balls or a hit by pitch;
- by taking a pre-existing baserunner's place on the bases as a result of a fielder's choice;
- by reaching base as a result of an error.
Between the time when a batter hits the ball and the time he reaches a base safely, he is known in the rules as the batter-runner.
How a baserunner advances to the next base
Once the batter-runner has reached a base and become a baserunner, there are a number of ways in which he can advance to the next base, which are described below.
A few fundamental rules apply:
- Baserunners must touch each base in succession (i.e. first base, then second base, then third base, then home plate), without skipping any base;
- There can never be two baserunners occupying the same base at one time; and
- Baserunners cannot pass one another.
Failure to obey any of these rules results in one of the baserunners being declared out, either automatically or on appeal.
Advancing on a batted ball
If a batter hits the ball in play, baserunners either can or must advance to the next base, depending on the situation:
- if the ball is hit on the ground, either as a ground ball or as a fly ball or line drive that falls to the ground, baserunners must advance to the next base in order to vacate a base for the batter-runner to occupy. In this case, a baserunner must reach the next base before the ball or he is out - this is known as a force play. If there is already an unoccupied base between him and the batter-runner, a baserunner is not forced to advance to the next base; if he still attempts to advance, he can only be put out by if a fielder applies a tag to him while he is not touching a base.
- if the ball is hit in the air and caught by a fielder before it hits the ground, the baserunner can only attempt to advance to the next base after the ball is caught. He must touch the base he is leaving after the ball is caught, and then arrive at the next base before the ball can be relayed there and a tag applied to him.
Advancing on a pitch
There are a number of ways a baserunner can advance to the next base on a pitch.
- He can attempt a stolen base, by leaving his base at the time the ball is pitched, and reaching the next base before the ball can be relayed to it by the catcher or another fielder and a tag applied to him. A runner can only attempt to steal an unoccupied base, unless two or more runners attempt to steal bases simultaneously, such as in a double steal;
- He can attempt to advance to the next base if the pitch gets away from the catcher, either as a result of a potential wild pitch or passed ball; in both cases, the fielders may still put him out by recovering the ball, throwing it to the base the baserunner is attempting to reach, and applying a tag.
- He can attempt to advance on an error, for example if the pitcher throws an errant pick-off attempt to any base, or if after a pitch, the catcher relays the ball back to the pitcher imprecisely.
- He can also advance to the next base without ability to be put out any time the pitcher commits a balk, or if the batter draws a base on balls or is hit by a pitch and there is no unoccupied base between the baserunner and first base.
Putting a baserunner out
There are a number of ways for the fielders to put a baserunner out. They include being out as a result of violating one of the three fundamental rules listed above; failure to advance to a base on batted ball, as described above; or failure to complete an attempted stolen base, or an attempted advance on a potential wild pitch or passed ball (all three of which are counted as a caught stealing).
In addition a baserunner can be put out in the following manners:
- by being away from his base when a batted ball is caught on the fly: he is out if he fails to return to the base before the ball arrives (no tag is required).
- by leaving his base before the ball is caught when trying to advance on a caught fly ball; in this case, the fielders need to appeal to the umpire.
- by being picked off a base by the pitcher or catcher.
- by being hit by a batted ball in fair territory.
- by leaving the basepath (a corridor of three feet to each side of a straight line linking two bases) in order to avoid a tag;
- by committing offensive interference.
Baserunners left on base
At the end of an inning, any baserunner that has neither been put out nor has scored is considered to have been left on base. A team can strand up to three baserunners in any inning. Runners left on base (usually abbreviated LOB) are lost to the offensive team, as every new inning begins with the bases empty.
A team that leaves a lot of runners on base will often be considered to lack clutch hitting ability. However, it must be noted that teams that are very weak in on-base percentage or that hit into a lot of double plays will leave few runners on base, while teams that place a lot runners on base will also strand many of these, but will usually score a lot of runs in the process. Therefore the number of runners left on base is not a statistic that is very revealing by itself.
- Dave Newhouse: "Sliding: A Lost Art in the Majors", Baseball Digest, November 1990, pp. 77-78. 
- Jim Reisler: "He May Be Fast, but Is He Quick ? Former Players Talk About Baserunning", in The Baseball Research Journal, SABR, Volume 38, Number 1 (Summer 2009), pp. 88-97.