What will the referee do if a basketball player foul?

An attempt to unfairly disadvantage an opponent through personal contact is illegal and is called a foul. These are most commonly committed by defensive players; however, they can be committed by offensive players as well. Players who are fouled either receive the ball to pass inbounds again, or receive one or more free throws if they are fouled in the act of shooting, depending on whether the shot was successful. One point is awarded for making a free throw, which is attempted from a line 4.5 metres (15 ft) from the basket.

The referee may use discretion in calling fouls (for example, by considering whether an unfair advantage was gained), sometimes making fouls controversial calls. The calling of fouls can vary between games, leagues and even between referees.

A player or coach who shows poor sportsmanship, for instance, by arguing with a referee or by fighting with another player, can be charged with a technical foul. The penalty involves free throws and varies between leagues. Repeated incidents can result in disqualification. Blatant fouls with excessive contact or that are not an attempt to play the ball are called unsportsmanlike fouls (or flagrant fouls in the NBA) and incur a harsher penalty; in some rare cases a disqualifying foul will require the player to leave the playing area.

If a team surpasses a preset limit of team fouls in a given period (quarter or half) – four for international and NBA games – the opposing team is awarded one or two free throws on all subsequent fouls for that period, the number depending on the league. A player who commits five fouls, including technical fouls, in one game (six in some professional leagues, including the NBA) is not allowed to participate for the rest of the game, and is described as having "fouled out".

After a team has committed a specified number of fouls, it is said to be "in the penalty". On scoreboards, this is usually signified with an indicator light reading "Bonus" or "Penalty" with an illuminated directional arrow indicating that team is to receive free throws when fouled by the opposing team. (Some scoreboards also indicate the number of fouls committed.)

The number of free throws awarded increases with the number of fouls committed. Initially, one shot is awarded, but after a certain number of additional fouls are committed the opposing team may receive (a) one shot with a chance for a second shot if the first shot is made, called shooting "one-and-one", or (b) two shots. If a team misses the first shot (or "front end") of a one-and-one situation, the opposing team may reclaim possession of the ball and continue play. If a team misses the first shot of a two-shot situation, the opposing team must wait for the completion of the second shot before attempting to reclaim possession of the ball and continuing play.

If a player is fouled while attempting a shot and the shot is unsuccessful, the player is awarded a number of free throws equal to the value of the attempted shot. A player fouled while attempting a regular two-point shot, then, receives two shots. A player fouled while attempting a three-point shot, on the other hand, receives three shots.

If a player is fouled while attempting a shot and the shot is successful, typically the player will be awarded one additional free throw for one point. In combination with a regular shot, this is called a "three-point play" because of the basket made at the time of the foul (2 points) and the additional free throw (1 point). Four-point plays, while rare, can also occur.