Officials

In serious play, there is an officiating head judge or chair umpire (usually referred to as the umpire), who sits in a raised chair to one side of the court. The umpire has absolute authority to make factual determinations. The umpire may be assisted by line judges, who determine whether the ball has landed within the required part of the court and who also call foot faults. There also may be a net judge who determines whether the ball has touched the net during service. In some tournaments, certain line judges, usually those who would be calling the serve, are replaced by electronic sensors that beep when an out call would have been made. In some open-tournament matches, players are allowed to challenge a limited number of close calls by means of instant replay. The U.S. Open, the Miami Masters, U.S. Open Series, and World Team Tennis started using a "challenge" system in 2006 and the Australian Open and Wimbledon introduced the system in 2007. This used the Hawk-Eye system and the rules were similar to those used in the NFL, where a player gets a limited number of instant-replay challenges per match/set. In clay-court matches, a call may be questioned by reference to the mark left by the ball's impact on the court surface.
The referee, who is usually located off the court, is the final authority about tennis rules. When called to the court by a player or team captain, the referee may overrule the umpire's decision if the tennis rules were violated (question of law) but may not change the umpire's decision on a question of fact. If, however, the referee is on the court during play, the referee may overrule the umpire's decision.
Ball boys or girls may be employed to retrieve balls, pass them to the players, and hand players their towels. They have no adjudicative role. In rare events (e.g., if they are hurt or if they have caused a hindrance), the umpire may ask them for a statement of what actually happened. The umpire may consider their statements when making a decision. In some leagues, especially junior leagues, players make their own calls, trusting each other to be honest. This is the case for many school and university level matches. However, the referee or referee's assistant can be called on court at a player's request, and the referee or assistant may change a player's call. In unofficiated matches, a ball is out only if the player entitled to make the call is sure that the ball is out.

Juniors

In tennis, a junior is any player under the age of 18 who is still legally protected by a parent or guardian. Players on the main adult tour who are under 18 must have documents signed by a parent or guardian. These players, however, are still eligible to play in junior tournaments.
The International Tennis Federation (ITF) conducts a junior tour that allows juniors to establish a world ranking and an Association of Tennis Professionals (ATP) or Women's Tennis Association (WTA) ranking. Most juniors who enter the international circuit do so by progressing through ITF, Satellite, Future, and Challenger tournaments before entering the main circuit. The latter three circuits also have adults competing in them. Some juniors, however, such as Australian Lleyton Hewitt and Frenchman Gael Monfils, have catapulted directly from the junior tour to the ATP tour by dominating the junior scene or by taking advantage of opportunities given to them to participate in professional tournaments.
In 2004, the ITF implemented a new rankings scheme to encourage greater participation in doubles, by combining two rankings (singles and doubles) into one combined tally. Junior tournaments do not offer prize money except for the Grand Slams, which are the most prestigious junior events. Juniors may earn income from tennis by participating in the Future, Satellite, or Challenger tours. Tournaments are broken up into different tiers offering different amounts of ranking points, culminating with Grade A.
Leading juniors are also allowed to participate for their nation in the Junior Fed Cup and Davis Cup competitions as well. To succeed in tennis often means having to begin playing at a young age. To facilitate and nurture a junior's growth in tennis, almost all tennis playing nations have developed a junior development system. Juniors develop their play through a range of tournaments on all surfaces, accommodating all different standards of play. Talented juniors may also receive sponsorships from governing bodies or private institutions.

Match Play
A tennis match is intended to be continuous. Stamina is a relevant factor, so arbitrary delays are not permitted. In most cases, service is required to occur no more than 20 (ITF events) or 25 (ATP and WTA events) seconds after the end of the previous point. This is increased to 90 seconds when the players change ends (after every odd-numbered games), and a 120 second break is permitted between sets. Other than this, breaks are permitted only when forced by events beyond the players' control, such as rain, damaged footwear, damaged racquet, or the need to retrieve an errant ball. Should a player be determined to be stalling repeatedly, the chair umpire may initially give a warning followed by subsequent penalties of "point," "game," and default of the match for the player who is consistently taking longer than the allowed time limit.
In the event of a rain delay or other such proponent, the match must be resumed at a later time. On junior professional circuits the matches are to be resumed at the score which was at the time of the delay. However, as per new revisions beginning with the 2006 Australian Open, the ATP and WTA govern different regulations regarding delays; in the event of a rain delay, the match will resume while only the end of the previously completed set before the delay is official.
Balls wear out quickly in serious play and, therefore, in ATP and WTA tournaments, they are changed after every nine games with the first change occurring after only seven games, because the first set of balls is also used for the pre-match warm-up. However, in ITF serious tournaments like Fed Cup the balls are changed in a 9-11 style. Continuity of the balls' condition is considered part of the game, so if a re-warm-up is required after an extended break in play (usually due to rain), then the re-warm-up is done using a separate set of balls, and use of the match balls is resumed only when play resumes.
It has recently been proposed to allow coaching on court during a match on a limited basis. Also, technological review of official calls made its debut in a major tournament at the 2006 U.S. Open.