A blocked shot is credited to a defensive player when the defender alters the shot by making contact with the ball after it has left the shooter's hands. If the shot does not go in the basket, this is a blocked shot.

Goaltending occurs when a defensive player interferes illegally with a shot on the rim or on the downward path to the hoop. The shot is assumed in and the offensive team receives the points. The defensive team takes the ball out of bounds and resumes play.

Coaches may instruct their defenders to guide, or funnel, their man to the basket because there is a human eraser, like centre Dikembe Mutombo, there to correct their mistakes. By playing off the man they are defending, shotblockers are able to help out defensively by coming across the lane to block and contest shots.

Dikembe Mutombo
Former Nets centre Dikembe Mutombo is one of the league's top shotblockers.
NBAE Photos
Having the luxury of a shotblocker allows perimeter defenders to be more daring as they will go for steals knowing the center is patrolling the lane.

A shotblocker provides a psychological advantage for his team by providing an intimidating presence to offensive players. The shotblocker may cause shooters to change their shots or concentrate on the shotblocker, not on their shots. A shooter's confidence and concentration may also be affected by the imposing defender, especially close to the basket.

One of the NBA's greatest players was the Boston Celtics' legendary defensive centre Bill Russell. Russell was a dominant force who controlled the game at the defensive end and his strong play transformed the Celtics into a championship-calibre club.

Russell would block shots but keep the ball in play for his teammates to recover. A Russell-blocked shot often led to a fast break or breakaway layup and the resulting four-point play (the two points the Celtics didn't give up, plus the two they scored off the break) frequently built the momentum the Celtics needed to win a game.

"It wasn't a good defensive play until you had possession of the ball going the other way," Russell explained. "Guys now are blocking the ball into the stands. All that does is delay the offence. It doesn't stop them."

There are many important factors to being a good shotblocker.

1. Timing: Shotblockers must be able to anticipate when the shooter will release the ball and then block it before it is on the way down, or else it is goaltending.
2. Quickness: Most blocks result from rotating to pick up a beaten teammate's man.
3. Height: Most of the NBA's blocked shots leaders are 6'10 or taller.
4. Being Left-Handed: Because most shooters are right-handed, left-handed defenders, such as Russell or Utah Jazz forward/centre Keon Clark, excel at blocking shots because the ball is on the same side as their shotblocking hand.
5. Jumping Ability: Great leapers such as Clark use their athletic talents to alter shots.