Obstruction describes an act by a fielder, who is not in possession of the ball or in the process of fielding it, that impedes the baserunner's progress.

If a play is being made on the obstructed baserunner, the ball is ruled dead and the umpire can place all runners on the base he determines they would have reached without the obstruction. If no play is being made on the obstructed runner, the umpire will allow the play to progress until its natural conclusion and then impose any penalties he sees fit to nullify the act of obstruction.