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.

Type A obstruction occurs when the obstructed runner is being played upon by the fielders. The ball is immediately dead, and all runners are awarded the base that the umpire judges the runners would have reached without the obstruction; however, the obstructed runner must be awarded at least one base. Umpiring practice grants a broad interpretation of whether a runner is being played upon; if the defense's actions as a whole are focused on a runner, he qualifies as being played upon for the purposes of determining the penalty for obstruction.

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.

Type B obstruction occurs when the obstructed runner is not being played upon. The ball does not become dead; rather, the umpire calls "That's obstruction!", but play is allowed to continue. While play continues, the umpire privately decides what base the obstructed runner would have reached without the obstruction. The obstructed runner is now "protected" until he reaches that base. When playing action stops, the ball will become dead and the runner will be awarded that base if he has not reached it. If he was put out before he reaches that base, that out will be nullified and he will be awarded that base. If the runner reaches that base safely, the obstruction is ignored. If the runner continues past that base, he does so at his own risk, without protection. There is no minimum base award for type B obstruction.

Under NFHS rules, all obstruction is considered type B; the ball does not become dead until after playing action ceases.

Notes:  If a fielder causes the runner to alter his normal running path, he can be guilty of obstruction (Contact is not necessary).  A fake tag is considered obstruction.  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.