Runner interference protects the fielder making the play.

How umpires check it

1. Identify the play being made: fielding a batted ball, taking a throw, making a throw, or handling a ball already deflected.
2. Decide which fielder, if any, was protected and entitled to make the play at that moment.
3. Judge whether the runner hindered that fielder, intentionally touched or deflected the ball, or violated the first-base running lane while interfering with a throw.
4. Check exceptions, including a legally occupied base, a ball that already passed an infielder, a deflected ball, or a fielder who was no longer protected.
5. Apply the result: usually a dead ball and an out on the interfering runner, with other runners returned unless forced or unless a specific rule creates a different placement.

What counts as runner interference

A runner interferes when the runner illegally changes the defence's chance to make a play. The most common form is hindering a fielder who is trying to field a batted ball. That can be contact, screening the fielder, stopping in the fielder's path, or otherwise preventing the fielder from making a normal attempt.

The call is not based only on whether the runner meant to interfere. On a batted ball, a runner who hinders the protected fielder can be out even if the runner was simply trying to advance. Baseball gives the fielder priority to field the batted ball.

When the fielder has right of way

When a batted ball is in play and a fielder is making a legitimate attempt to field it, the runner must avoid hindering that fielder. If two fielders could make the play, umpires protect the one they judge had the best play on the ball. A runner who impedes an unprotected fielder may not be guilty of interference if another fielder was the one actually entitled to the play.

This is why timing matters. Before the ball is fielded, the fielder usually owns the space needed to make the play. After the ball is fielded or missed and the fielder is no longer making the initial play, the runner may regain ordinary baserunning rights unless another rule applies.

Fair batted balls that touch a runner

A runner is generally out if touched by a fair batted ball in fair territory before the ball has gone through or by an infielder, and no other infielder has a chance to make a play. The ball becomes dead, the batter is usually credited with reaching first, and runners advance only if forced.

If the ball has already passed an infielder other than the pitcher and no other infielder has a realistic play, the runner is usually not out just because the ball touches the runner. A deflected ball can also change the ruling because the initial fielding chance may already have been used.

A base is protection, but not a shield for intent

A runner touching a legally occupied base is often protected if contact with the fielder happens during a play on a batted ball near that base. That exception prevents a runner from being punished simply for being where the runner is legally allowed to be.

The exception is limited. A runner may not deliberately hinder the fielder, kick or redirect the ball, wave arms to screen the fielder, or use the base as cover for an intentional act. Intentional interference can still be called even if the runner is standing on the base.

Interference with throws usually requires intent

A runner is not automatically out because a thrown ball hits the runner. Throws often hit runners during normal play, especially in rundowns or near first base. For a thrown-ball interference call, umpires usually look for an intentional act: reaching, swatting, kicking, changing path to be hit, or otherwise clearly trying to affect the throw.

There are separate rules for illegal slides and double-play takeout actions, where a runner can interfere by initiating improper contact or failing to use a bona fide slide. Those plays are closely related but are not every ordinary thrown-ball contact.

The batter-runner going to first

The first-base running lane rule is a common runner interference call. In the last half of the distance to first base, the batter-runner must use the legal lane area when a throw is being fielded to first. If the batter-runner runs outside the permitted area and interferes with the fielder taking the throw, the batter-runner is out and the ball is dead.

The runner is still allowed to leave the lane when necessary to touch first base or to avoid a fielder trying to field a batted ball. The violation is not being outside the lane by itself; it is being outside the permitted area and interfering with the play at first.

What happens after the call

For most runner interference on a batted ball, the ball is dead immediately and the interfering runner is out. Other runners usually return to the bases they last legally occupied at the time of interference, unless forced by the batter becoming a runner or unless the rule gives a different result.

If the interference is designed to prevent a double play, umpires can call an additional out. On double-play slide violations, the runner and batter-runner can both be declared out. The purpose is to remove the advantage gained by illegally stopping the defence from completing the play.

Legal contact still happens

• Ball hits the runner after passing the infielders: if no infielder has a remaining play, it is usually live-ball contact rather than interference.
• Throw hits a runner in a normal running path: without an intentional act or a specific lane violation, the play usually continues.
• Fielder moves into the runner after missing the ball: once the fielder is no longer protected, ordinary obstruction and baserunning rules may matter instead.
• Runner is on a legally occupied base: accidental contact near that base may be protected unless the runner intentionally interferes.

"Contact means interference"

Contact is evidence, not the rule. A runner can interfere without contact by blocking a fielder's path or view, and a runner can be hit by a throw without committing interference. The umpire is judging hindrance, right of way, and intent where the rule requires intent.

"The runner always owns the baseline"

The baseline does not give a runner priority over a fielder trying to field a batted ball. On that kind of play, the runner must avoid the protected fielder. The runner's path matters for tags and lane calls, but it does not erase the fielder's right to field the ball.

Why local games may be stricter

Professional-style rules distinguish between accidental contact, ordinary baserunning, interference on batted balls, intentional interference with throws, running lane interference, and illegal slides. Youth, school, college, and local leagues may add stricter safety language for malicious contact, avoidable contact, verbal interference, or ejections.

For a real game, use the rule book adopted by that competition. The core idea is stable across baseball, but the penalty, dead-ball procedure, appeal mechanics, and discipline can vary by level.

Four quick rulings

• Runner from second collides with the shortstop fielding a ground ball: runner interference, runner out, ball dead, other runners return unless forced.
• Ground ball passes the third baseman, no other infielder has a play, and then hits the runner: usually no interference; the ball stays live.
• Batter-runner runs well inside the lane and blocks the first baseman's chance to catch a throw: running lane interference, batter-runner out, ball dead.
• Runner in a rundown is hit by a thrown ball while running normally: usually live ball unless the runner intentionally tried to be hit or otherwise hindered the play illegally.

Source material

• MLB Official Information
• Official Baseball Rules: Rules 5.09(a)(11), 5.09(b)(3), 5.09(b)(7), 6.01(a), 6.01(b), and 6.01(j)
• MLB Obstruction Glossary
• Little League Rules, Regulations, and Policies