They’re two opposite fouls people mix up. In short: interference is committed by the offense when it gets in the defense’s way, and obstruction is committed by the defense when it blocks a runner without having the ball.
Interference (offense’s foul)
It happens when a batter or runner keeps a fielder from making a play: for example, a runner who barrels into the pivot man on purpose, or a batter who gets in the catcher’s way on a throw to second. The usual penalty is an out for whoever interfered.
Obstruction (defense’s foul)
It’s the reverse: a fielder who doesn’t have the ball blocks a runner’s path. The classic is a first or third baseman standing in the base path without taking a throw. The umpire awards the runner the base he would have reached, so the foul doesn’t hurt him.
FAQ
How do I tell them apart quickly?
Ask who got in the way: if it was the offense, it’s interference; if it was the defense without the ball, it’s obstruction.
Can the umpire wipe out runs?
Yes: depending on the case, he can call outs, award bases or adjust the play to undo the foul.
