Most recreational matches and junior tournaments are umpired by the players.
But in professional competitions, officials keep the score, judge whether the ball is in and interpret rules.
The senior umpire, who calls the score and can overrule all other officials, sits in a tall chair at one end of the net.
There are also line judges who sit outside the court looking down all the court lines. It is their job to decide whether a ball is in or out.
From the 1980s, when players' serves began to regularly top 100mph, the service line judge has had the help of an electronic machine.
It's known as Cyclops, after the one-eyed giant of ancient Greek legend.
There may also be a net-cord judge to determine whether a serve has clipped the net and should be called a let.
Players who argue with tournament officials can be penalised points and in extreme cases disqualified.
In the event of a major dispute, the umpire can call on the match referee who watches play from the stands.