A referee presides over a game of association football. The referee has "full authority to enforce the Laws of the Game in connection with the match to which he has been appointed" (Law 5), and the referee's decisions regarding facts connected with play are final, so far as the result of the game is concerned.
The referee is assisted by two assistant referees (formerly known as linesmen), and in some matches also by a fourth official. In 2006, the appointment of a fifth official also became possible, due to implementation by FIFA. The match officials utilise a positioning system known as the diagonal system of control.
The vast majority of referees are amateur, though they are usually paid a small fee and/or expenses for their services. However, in some countries a limited number of referees - who mainly officiate in their country's top division - are employed full-time by their national associations and receive a retainer at the start of every season plus match fees.
Referees are licensed and trained by the same National organizations that are members of FIFA. Each National organization recommends its top officials to FIFA to have the additional honor of being named a FIFA official. International games between National teams require FIFA officials. Otherwise, the local National organization determines the manner of training, ranking and advancement of officials from the youngest youth games through professional matches.
- enforcing the Laws of the Game;
- controlling the match in co-operation with the assistant referees and, where applicable, with the fourth official;
- ensuring that any ball used meets the requirements of Law 2;
- ensuring that the players' equipment meets the requirements of Law 4;
- acting as timekeeper and keeping a record of the match;
- stopping, suspending or terminating the match, at his discretion, for any infringements of the Laws;
- stopping, suspending or terminating the match because of outside interference of any kind;
- stopping the match if, in his opinion, a player is seriously injured and ensuring that he is removed from the field of play. An injured player may only return to the field of play after the match has restarted;
- allowing play to continue until the ball is out of play if a player is, in his opinion, only slightly injured.
Referees use a whistle to indicate the commencement or restart of play, to stop or delay play due to an infringement or injury, or to indicate that time has expired in the half. The whistle is an important tool for the Referee along with verbal, body and eye communication. The use of whistles is not mandated by the Laws of the Game.
In fact, the whistle was not mentioned in the Laws of the Game (LOTG) until very recently. The main LOTG simply mentions the referee should signal certain events. Only in 2007, when the IFAB greatly expanded the LOTG Additional Instructions section, did they mention the whistle. In fact, they wrote up a full page of advice on how and when the whistle should be used as a communication and control mechanisms by the Referee.
Before the introduction of the whistle, referees indicated their decisions by waving a handkerchief. The whistles that were first adopted by referees were made by Joseph Hudson at Mills Munitions in Birmingham, England. The ACME Whistle Company (based at Mills Munitions Factory) first began to mass produce pea whistles in the 1870s for the Metropolitan Police Service. It is frequently stated the referee's whistle was first used in a game between Nottingham Forest and Sheffield Norfolk in 1878; however no such fixture is known to have taken place between the two clubs in that year.
Modern day referees and their assistants wear a uniform consisting of a jersey, shorts and socks: until the 1950s it was more common for a referee to wear a blazer than a jersey. Traditionally that uniform was almost always all black, unless one of the teams was wearing a very dark jersey in which case the referee would wear another colour of jersey (usually red) to distinguish himself from both teams. At the 1994 World Cup finals, new jerseys were introduced that gave officials a choice of burgundy, yellow or white, and at the same time the creation of the FA Premier League in England saw referees wear green jerseys: both changes were motivated by television considerations. Since then, most referees have worn either yellow or black, but the colours and styles adopted by individual associations vary greatly. For international contests under the supervision of FIFA, Adidas uniforms are worn because Adidas is the current sponsor. FIFA allows referees to wear five colours: black, red, yellow, blue and green.