They say that the men and women in black are having a good game if you don't notice them.
It's the one with the whistle - the referee - who takes charge of a match, with the help of two assistant referees.
In professional matches, a fourth official is also involved.
The referee enforces the laws of the game, awarding free-kicks if there is any foul play and keeping a check on the time.
The ref can also postpone, stop, suspend or call off a match if there are weather or crowd problems.
The assistant referee helps out with decisions such as throw-ins and offsides and sometimes will have a better view of incidents than the referee.
The fourth official is based on the touchline, assisting with substitutions and keeping a check on the managers.