In baseball, when a fielder illegally hinders a baserunner, the fielder is guilty of obstruction.

Baserunners are generally permitted the free privilege to run from base to base without being physically blocked or hindered by a fielder. The only time a fielder need not "get out of the way" of a baserunner is when the fielder is fielding or in possession of the ball.

Penalty

Obstruction falls into one of two categories, generally called type "A" and type "B", after the subsection of Official Baseball Rule 7.06, in which each type of obstruction is described.

Type A obstruction occurs when the obstructed runner is being played upon by a 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.

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

Common misconceptions

There need not be physical contact for obstruction to be called; rather, if a fielder causes the runner to alter his normal running path, he can be guilty of obstruction.

Obstruction is not to be confused with interference. When a fielder hinders a baserunner, many baseball commentators will mistakenly refer to obstruction as interference.