Obstruction is called when the defense hinders the runners ability to run the bases. There are two different applications of the rule. One causes an immediate dead ball and the other is delayed dead. If a play is being made on a runner who is obstructed, the ball is immediately dead. If no play is being made the ball is delayed dead. A play for purposes of this rule is when the ball is in-flight heading toward the base the runner is heading, an attempted tag, or when the runner is caught in a run-down. The rule book definition is:

OBSTRUCTION is the act of a fielder who, while not in possession of the ball and not in the act of fielding the ball, impedes the progress of any runner.

If a fielder is about to receive a thrown ball and if the ball is in flight directly toward and near enough to the fielder so he must occupy his position to receive the ball he may be considered "in the act of fielding a ball." It is entirely up to the judgment of the umpire as to whether a fielder is in the act of fielding a ball. After a fielder has made an attempt to field a ball and missed, he can no longer be in the "act of fielding" the ball. For example: an infielder dives at a ground ball and the ball passes him and he continues to lie on the ground and delays the progress of the runner, he very likely has obstructed the runner."

A fake tag is considered obstruction. The fielder may stand in the base path without the ball IF the throw is almost to him and he needs to be there to catch the ball. "Almost to him" is a judgment by the umpire. Some say that when a throw is over the infield grass and heading toward the fielder; the fielder is "in the act of fielding" and may stand where he needs to, to catch the ball. However, he may not actually block the base until he has possession of the ball. Until he has possession of the ball he must give the runner some way to get to the base. Obstruction can NEVER be called on a fielder for blocking a base; when he has possession of the ball.

As with interference, obstruction is also a tough judgment call. Contact between the runner and fielder is not necessary to meet the definition. If a runner must slow down or alter his path to avoid a fielder who is not in possession of the ball and not in the act of fielding, he has been obstructed.

If no play is being made on the runner at the time he is obstructed, the play continues. The tough part comes when the play stops. The umpire will award the runner the base to which the umpire believes he would have reached had he not been obstructed. For example: the batter hits a ball in the gap for what looks like an easy double. No play is being made on him. As he rounds first the fielder is in his path and they collide. The batter stops at first. The umpire will award the runner second base if he believes the runner would have made it there had he not been obstructed.

It does not matter where the obstruction occurs. If a runner is obstructed at first base and the umpire believes he could have made it to third base, he will be awarded third. The umpire must be the judge. If, in the umpire's judgment, a runner is slowed down by one step at first and then is thrown out by five steps at third, the out should stand.

An immediate dead ball obstruction is called when obstruction occurs while a play is being made on the runner. For example: a runner on first is attempting to reach third on a hit. He is obstructed by a fielder between second and third as the throw from the outfield is heading toward third. This is a play on the runner. The umpire should call "time" when the obstruction occurs and award the runner third base. Another example is a run-down play. It does not matter which way the runner is heading. If he is obstructed while being played upon in a run-down, he is awarded at least one base beyond the last base he held.

If a runner is obstructed attempting to get back to first on a pick-off play, the ball is dead and he is awarded second.

If a runner is awarded bases due to obstruction; runners ahead of him are forced to the next base. However, trailing runners are not always given another base when obstruction awards a lead runner another base. Rule 7.06 covers obstruction. 7.06(a) is when a play is being made and 7.06(b) is when there is no play being made.