According to the International Tennis Federation, a foot fault is an infraction that occurs during a player's serve. In an official match, foot faults will result in the player losing the point in which the fault occurred.
According to the International Tennis Federation, a player can be charged with a foot fault if any of the following occur:
1. The player changes position by walking or running, although slight movements of the feet are permitted
2. The player touches the baseline or the court with either foot
3. The player touches the area outside the imaginary extension of the sideline with either foot
4. The player touches the imaginary extension of the center mark with either foot
In official matches, a line judge is responsible for calling a foot fault. In non-official matches, the person receiving the serve usually calls the infraction.