A foul is a shot or action by the striker which is against the rules of the game.

When a foul is made during a shot, the player's turn is ended and he will receive no points for the foul shot. The other player will receive penalty points.

Common fouls are:

failing to hit any other ball with the cue ball

first hitting a ball "not-on" with the cue ball

potting a ball "not-on"

potting the white (in-off)

hitting a ball other than the white with the cue

making a ball land off the table

touching the cue ball with something other than the tip of the cue

playing a "push shot" - a shot where the cue, cue ball and object ball are in simultaneous contact

playing a "jump shot", which is where the cue ball leaves the bed of the table and jumps over a ball (even if touching it in the process) before first hitting another ball

playing a shot with both feet off the ground

Should a cue ball be "in-hand", i.e. when breaking-off or playing from the "D" after being potted, be touched with the tip, a foul is not committed as long as the referee is satisfied that the player was only positioning the ball, and not playing, or preparing to play, a shot.

When a foul is made, the other player will receive penalty points. Penalty points are at least 4 points and at most 7 points. The number of penalty points is the value of the ball "on", or any of the "foul" balls, whichever is highest. When more than one foul is made, the penalty is not the added total — only the most highly valued foul is counted.

Not hitting the ball "on" first is the most common foul. Players can make life difficult for an opponent by making sure that they cannot hit a ball "on" directly. This is most commonly called "snookering" one's opponent or alternatively "laying a snooker" or putting the other player "in a snooker".

Since players receive points for fouls by their opponents, snookering one's opponent a number of times in a row is a possible way of winning a frame when potting all the balls on the table would be insufficient for you to win.

If a player commits a foul, and his opponent considers that the position left is unattractive, he may request that the offender play again from that position.

If a foul has been committed by not hitting a ball "on" first, or at all, and the referee judges that the player has not made the best possible effort to hit a ball "on", and neither of the players are in need of snookers to win the frame, or would be after the penalty points were applied, then "foul, and a miss" is called. In this instance the other player may request that all balls on the table are returned to their position before the foul, and the opponent play the shot again. (In top class play, this will usually require only the cue ball and a couple of other balls to be moved.) This rule is often applied less stringently, if at all, in amateur matches.

Since jump shots are explicitly forbidden, it can happen that the ball on is physically impossible to hit even with a swerve shot. (This can happen, for instance, if the cue ball is crowded into a corner and closely surrounded by several reds after a red was potted. It is technically possible to drive one's opponent into a physically impossible snooker by dislodging several colours in one shot, but this is virtually unheard of.) In this case the player is nevertheless required to nominate a ball and attempt to hit it, i.e. to play in the direction that would hit it if the obstacles were not present. A foul shot will inevitably be called, but since the task was impossible, this will never be a miss, and the game continues normally.

When a player leaves an opponent unable to hit both lateral extremities of at least one ball "on" after a foul, the opponent will receive a free ball. This means any colour can be nominated and played as the ball "on". The number of points for potting the free ball is not the worth of the nominated ball but of the original ball "on". For example, if the ball "on" is a red, and the free ball is a pink, the player will receive one point for potting the pink. After potting the free ball as a red, a player can nominate and pot a colour as usual. The only exception to this rule is if the "on" ball is a red that is being obstructed by another red. For example, if the player breaking-off hits the white too soft and it does not hit the triangle of reds, a free-ball would not be called, even though no red can be hit on both sides, since the only thing preventing some of the reds being hit on both sides is another red.