In basketball, a block or blocked shot happens when a defensive player legally moves the ball away from an offensive player during a shot to stop a score. The defender must not touch the offensive player's hand unless they are also touching the ball, or a foul will be called. For a block to be legal, it must occur while the ball is going up or at its highest point. If a shot is deflected but still goes into the basket, it is not considered a blocked shot and is counted as a successful field goal. For the shooter, a blocked shot is recorded as a missed attempt. On a shooting foul, a blocked shot cannot be counted, even if a different player deflects the shot. If the ball is moving downward when the defender hits it, it is ruled as goaltending, and the shot counts as made. Goaltending is also called if the block happens after the ball bounces off the backboard (except in NFHS rules; NCAA used this rule until the 2009–10 season).

Blocked shots are sometimes called "rejections," "stuffs," "bushed," "spoinked," "fudged," "double-fudged" (two-handed blocks), "facials," "swats," "denials," or "packs." Blocked shots were first officially recorded in the NBA during the 1973–74 season.

Centers and power forwards often record the most blocks because of their height and position near the basket. However, shorter players with strong jumping ability can also block shots, such as Dwyane Wade, who was the shortest player (6'4") to block 100 shots in one season. A player who can block shots helps a team's defense by making it harder for opponents to shoot near the basket. If the blocked ball stays in play, it can lead to a fast break, a skill Bill Russell was known for. To block shots effectively, a player needs good court awareness, timing, and either height or jumping ability. One strategy is to intimidate opponents into changing their shots, which can cause them to miss.

Chase-down block

A chase-down block happens when a player runs to catch up with an opponent who has moved ahead of the defense, such as during a quick play. The player then blocks the opponent’s attempt to shoot the ball. This often happens when the opponent is trying to score near the basket, and the blocking player hits the ball into the backboard. One famous example is when Tayshaun Prince of the Detroit Pistons blocked Reggie Miller’s shot in Game 2 of the 2004 NBA Eastern Conference Finals against the Indiana Pacers. This block helped his team win the game. Fred McLeod, a Pistons announcer, first saw Prince perform this type of block and later helped create the term "chase-down block" while working with the Cleveland Cavaliers. During the 2008–09 NBA season, the Cavaliers started recording chase-down blocks, giving LeBron James credit for 23 in that season and 20 in the next season. Another important chase-down block happened in the 2016 NBA Finals. LeBron James blocked Andre Iguodala’s lay-up attempt in the final minutes of the 4th quarter when the score was tied at 89–89 with 1 minute and 50 seconds left in the game.

Shot-blocking records in the NBA

• Most blocks in one game: Elmore Smith (17)
• Most blocks in one half of a game: Elmore Smith, George T. Johnson, Manute Bol (11 each)
• Highest average blocks per game during a season: Mark Eaton (5.56)
• Most blocks in a player’s career: Hakeem Olajuwon (3,830)
• Highest average blocks per game in a career: Mark Eaton (3.50)
• Most blocks in an NBA Finals game: Dwight Howard (9)
• Most blocks in a playoff game that was not part of the NBA Finals: Andrew Bynum, Hakeem Olajuwon, Mark Eaton (10 each)

Shot-blocking records in NCAA Division I

• Most career blocks: Jarvis Varnado from Mississippi State with 564 blocks from 2006 to 2010
• Most blocks in one season by a player: David Robinson from Navy with 207 blocks in 1985–86
• Most blocks per game in one season by a player: Shawn James from Northeastern with an average of 6.53 blocks per game in 2005–06
• Most blocks in one season by a team: Kentucky with 344 blocks in 2011–12

• Most career blocks: Brittney Griner from Baylor with 736 blocks from 2009 to 2013
• Most blocks in one season by a player: Brittney Griner from Baylor with 223 blocks in 2009–10
• Most blocks per game in one season by a player: Brittney Griner from Baylor with an average of 6.4 blocks per game in 2009–10
• Most blocks in one season by a team: Baylor with 310 blocks in 2011–12