Often, parents and coaches alike incorrectly assess the occurrence of obstruction on a play, and may even refer to the perceived infraction as interference.

The purpose of this is to define ‘Obstruction’ and clarify how it differentiates from ‘Interference’.
 
What is Obstruction: Obstruction occurs when a fielder illegally hinders a baserunner within the basepath.  Obstruction is never called on the offense, but is instead an infraction against the defense. Another key point is that the fielder illegally hinders the baserunner, which is a judgement call to the umpire.

Why the Confusion: There is a faulty assumption that the offense has the right to a base, even at the detriment of a defender making a play on a ball. Due to this faulty assumption, parents/coaches will claim a defensive player interfered with the runner.

 Clarification: A defender has a right to make a play on a ball. To the extent possible, a runner must avoid ‘interfering’ with the defender. Obstruction occurs when a defensive player that is not making a play on the ball impedes the runner’s path to the next base.

 Examples of Obstruction: 

• Situation: Runner on second, one out, ball hit to second. As the runner proceeds to third, the shortstop impedes the runner, thereby allowing the second baseman to get the out at first and them permitting the first basemen to throw the runner out on third. The ruling should be runner out at first, and runner safe at third. If the umpire declares that the runner had a strong likelihood of making it to home on the play, the umpire can award home to the runner.
  
  

• Situation: Runner on second, ball hit to right field. As the runner nears third, the third baseman fakes a tag on the runner, though the throw is not coming to the base. As a result, the runner slows down and does not attempt to proceed home. This is a case of obstruction, as fake tags are not permitted. If t is in the umpires judgment that the runner would have made it home had a fake tag not been applied, he may award home to the runner.

• Situation: Runner on first, ball hit to right field. As the runner near second, the shortstop blocks the bag before the ball is thrown to him, thereby leading the runner to go around the player and touch the base on the other side. As the runner tries to touch the base, the ball is thrown to the shortstop, who in turn tags the player out. The ruling is that the runner will be awarded second base. The fielder can only stand in the basepath if the ball is almost to him, and the player needs to move into the basepath to catch the ball. The umpire is not likely to award third because if the play would have continue uninterrupted without the blockage, the player likely would have been thrown out.

• Situation: Runner on second, ball hit to shortstop. The shortstop does not field the ball cleanly, and instead deflects the ball into the basepath. As the fielder attempts to retrieve the ball, he knocks the runner over and proceeds to tag him out after retrieving the ball. The ruling is that the runner will be awarded third base. Once a defender misses or deflects a ball, their initial attempt ends and the runner has the right to the basepath.

Applicable Baseball Rule(s) Relating to Obstruction: 7.06(a) and 7.06(b)

Following is an obstruction call that took place in game 3 of the 2013 MLB World Series (Red Sox/Cardinals):