Law 42 of the laws of the sport of cricket covers fair and unfair play. This law has developed and expanded over time as various incidents of real life unfair play have been legislated against.
The first section of law 42 makes clear that the captains of the two teams have the responsibility for ensuring that play is conducted according to the spirit and traditions of the game, as well as within its Laws. This leads to a statement that the umpires are the sole judges of fair and unfair play. It contains an override of the laws of cricket: if either umpire considers an action that is not covered by the laws to be unfair, he can intervene and call the ball dead.
The state of the ball affects deliveries for a batsman. Even a new cricket ball is not perfectly spherical, but in two parts stitched together to form a seam. How a ball moves depends in part on how much air resistance there is to different parts of the ball, and therefore to what degree the ball has deteriorated. A cricket team will normally seek, for example, to polish one side of the ball and rough up the other side. The resulting variation in air resistance on the ball can have a marked effect.
Ball tampering has always been a feature of the sport. Players would use objects to rough up one side of the ball, and use resins and substances like Brylcreem to shine the other. This sort of ball tampering is against the spirit of the game and has always been against the rules. Despite this, it has always gone on with limited sanction, and it can be difficult to spot.
The television age has meant that from the 1990s onwards most international games have been televised. Slow motion replays have highlighted a number of incidents of ball tampering: some of which have been widely reported in the press. The third section of law 42 contains the rules and sanctions against ball tampering and requires the umpires to make frequent and irregular inspections of the ball to counter it. It also contains punitive measures against fielders who do tamper with the ball. Match suspensions may be implemented.
Some acts that may alter the ball are permitted. A fielder may polish the ball as long as no artificial substance is used, remove mud from the ball under the supervision of the umpire and dry a wet ball on a towel. But no-one may rub the ball on the ground for any reason, interfere with any of the seams or the surface of the ball, use any implement, or take any other action whatsoever which is likely to alter the condition of the ball.