The video assistant referee (VAR) is a football assistant referee who reviews decisions made by the head referee with the use of video footage and a headset for communication. In 2018, VARs were written into the Laws of the Game by the International Football Association Board (IFAB) following trials in a number of major competitions.
There are 4 types of calls that can be reviewed.
The standard for overturning the referee's original decision is that there has been a "clear error," sometimes expanded to "clear and obvious error."
The process begins with the video assistant referee(s) and the assistant video assistant referee (AVAR) reviewing the play in question on a bank of monitors in the video operation room (VOR) with the assistance of the replay operator. This can be triggered by the referee requesting the review or by the VAR conducting a "check" to see if a review should be recommended to the referee. If the VAR finds nothing during the check, then communication with the referee is unnecessary, which is called a "silent check". If the VAR believes there has been a potential clear error, the referee will be contacted with that judgment. The referee can then either (a) change the call on the advice of the VAR or (b) conduct an on-field review (OFR) by going to a designated spot on the sideline, called the referee review area, to review the video with the help of the review assistant or (c) decide that he/she is confident in the original call and not conduct an OFR. The referee is allowed to stop play to reverse a call or conduct an OFR, but is not supposed to do so when either team is engaged in good attacking possibility.[additional citation(s) needed]
The official signal for a video review is the referee making the outline of a rectangle with his index fingers (indicating a video screen). This precedes both any OFR as well as any change in the original call. Players who demand a video review by making the rectangle motion excessively are to be cautioned with a yellow card. Players who enter the area where the referee conducts an OFR are also to be cautioned with a yellow card, and team officials who do so are to be dismissed.
There are guidelines the referee and the VAR should follow in conducting a video review. For example, slow motion should only be used for "point of contact" offences, such as physical offences and handballs. Regular speed should be used to determine the intensity of an offence and whether a handball was deliberate. Reviews for goals, penalty kick decisions, and red cards for denial of an obvious goal scoring opportunity cover the period back to the beginning of the "attacking possession phase", when the attacking team first gained possession of the ball or restarted play. Other reviews only cover the incident itself.
The VAR will be a current or former referee.
The assistant video assistant referee (AVAR) is a current or former referee appointed to assist the VAR in the video operation room. The responsibilities of the AVAR include watching the live action on the field while the VAR is undertaking a "check" or a "review", to keep notes of incidents, and to communicate the outcome of a review to broadcasters.
VAR was conceived by the Refereeing 2.0 project in the early 2010s, under the direction of the Royal Netherlands Football Association (KNVB). The system was tested through mock trials during the 2012–13 season of the Eredivisie, the country's top football league. In 2014, the KNVB petitioned the International Football Association Board (IFAB) to amend its laws of the games to allow the system to be used during more extensive trials. The IFAB approved trials and a pathway to full implementation during its 2016 general meeting. Lukas Brud, IFAB secretary, said "With all the 4G and Wi-Fi in stadia today...we knew we had to protect referees from making mistakes that everyone can see immediately", such as Thierry Henry’s handball that eliminated Ireland from qualifying for the 2010 FIFA World Cup where the on-field referees were not in a position to view the infraction. Then-FIFA president Sepp Blatter, who was strongly against introducing new technology in football, was forced out of his post due to a corruption scandal in 2015, and the VAR proposal received a warm reception under his successor Gianni Infantino.
A live trial of the VAR system began in August 2016 with a United Soccer League match between two Major League Soccer reserve sides. Match referee Ismail Elfath reviewed two fouls during the match and, after consultation with video assistant referee Allen Chapman, decided to issue a red card and a yellow card in the respective incidents. Video reviews were introduced the following month during an international friendly between France and Italy. A "pitchside monitor" was introduced at the 2016 FIFA Club World Cup, allowing referees to review footage from the field.
The A-League in Australia became the first to use a VAR system in a top flight professional club competition on 7 April 2017, when Melbourne City played Adelaide United though this game was completed without the VAR being called upon. The first intervention by a VAR in a professional league game was seen on 8 April when Wellington Phoenix hosted Sydney FC. The VAR identified an illegal handball in the penalty area and awarded Sydney FC a penalty. The game finished in a 1–1 draw. Major League Soccer in the United States introduced VARs in competitive matches during its 2017 season after the 2017 MLS All-Star Game on 2 August 2017. Its first official use came during a match between the Philadelphia Union and FC Dallas, invalidating a goal from the latter over contact made between a Dallas player and Philadelphia's goalkeeper. VAR was used at an international level in the 2017 FIFA Confederations Cup in June, where it was praised but its usefulness was questioned after a referee decision in the final match.
The VAR system was introduced in top flight European football by Bundesliga and the Serie A at the beginning of the 2017–18 season and by La Ligue 1 at the beginning of the 2018–19 season. The system was also used at the 2017 FIFA U-20 World Cup in October. On 8 January 2018, VAR was trialled for the first time in England in the 2017–18 FA Cup game between Brighton & Hove Albion and Crystal Palace., and the following day it was trialled for the first time in France in the Côte d'Azur derby game in the 2017–18 French League Cup. It was said to have worked well.
On 3 March 2018, the IFAB wrote the VARs into the Laws of the Game on a permanent basis.  Their use remains optional for competitions, and the English Premier League and the UEFA Champions League are not expected to implement VAR for their 2018–19 seasons. However Premier League executive chairman Richard Scudamore has described it as "inevitable" that VAR will be introduced to the Premier League. Although VAR has not been implemented in the group stages of the 2018-19 Champions League season, UEFA are examining the possibility of utilising VAR in the knockout stages which commence in February 2019.
FIFA officially approved the use of VAR for the 2018 FIFA World Cup during the FIFA Council meeting on 16 March 2018 in Bogotá. This tournament became the first competition to use VAR in full (at all matches and in all venues).
The 2018 FIFA World Cup marked the system's World Cup debut. A total of 335 incidents were checked by the VAR over the course of the group stage, averaging seven per match, and fourteen calls made by referees were changed or overruled after being reviewed by the VAR. According to FIFA, the VAR system had a success rate of 99.3 percent, up from the 95 percent of correct calls by referees without VAR. The first VAR decision at the World Cup came on 16 June in a group stage match between France and Australia, where referee Andres Cunha awarded a penalty to France after consulting with the VAR. In the final, referee Néstor Pitana used the VAR to review a defensive foul for handling in the penalty area, awarding France a penalty, which gave them a 2–1 lead over Croatia; the final eventually ended with France prevailing 4–2.
The use of VAR has been cred with assisting the 2018 ion's status as the cleanest World Cup since 1986, after no red cards are issued in the opening 11 games and only four players were sent off in the entire tournament which was the fewest since 1978. 22 goals were scored from 29 awarded penalty spot kicks, beating the previous record of 17 penalty kick goals set in the 1998 FIFA World Cup; the dramatic increase in the number of penalties awarded at the 2018 World Cup has been attributed to VAR catching fouls which would otherwise have remained unpunished. International Football Association Board technical director David Elleray stated a belief that the presence of VAR meant that players would know that they would not be able to get away with anything under the new system.
The use of video technology at the 2017 FIFA Confederations Cup was criticised after several contentious moments involving VAR at the tournament. It was accused of "creating as much confusion as clarity".
Further criticism was leveled at VAR after it suffered issues preventing its use, for example in a Portuguese match where a supporter's flag had been obscuring the VAR camera, or in the 2018 A-League Grand Final between Newcastle Jets FC and Melbourne Victory FC where the VAR software suffered a technical malfunction which prevented the assistant referee from viewing the replay.
After the introduction of VAR in the 2018 World Cup, FIFA deemed it a success. Nevertheless, the use (or lack of use) of VAR has been criticised. Independent assessments note that while most decisions were made correctly as a result of VAR, some were wrong despite VAR review and some decisions which were called incorrectly were not even reviewed. The Guardian concludes that VAR has been most effective for factual decisions such as offsides and mistaken identities, while subjective decisions such as penalties or the disciplining of players have fared much worse. Lack of clarity and consistency are two main areas of weakness.
A problem in consistency at the World Cup 2018 was for example the different rulings in similar game situations, which could be explained by unclear interpretation of VAR rules. For instance, in the game between Portugal and Iran in the group stage, Iran got a penalty kick after a handball by Cedric Soares, while in the game between Nigeria and Argentina, Nigeria did not get this chance after Marcos Rojo headed the ball onto his own arm. 
Another line of criticism has been targeted at the effectiveness of the system in achieving its goal. In the opinion of Scott Stinson from the National Post, VAR, like any other replay system, fails to correct human error and instead only adds to the controversies because human judgment is still necessary. Lack of transparency is another contentious point, as teams have no way to know which incidents were reviewed by the VAR team. At a press conference held after the group stage, FIFA referees committee chairman Pierluigi Collina showed footage of the decision-making process accompanied with audio of the conversations between VAR officials and the referees. Asked if this audio could be made publicly available, as it is in rugby and cricket, Collina answered enthusiastically but cautioned that it might still be too early.
Others have pointed to the game-changing nature of VAR. Initial fears that using the system would lengthen the game considerably have not been confirmed, with every VAR review taking up an average of only 80 seconds. The dramatic increase in the number of penalties awarded at the 2018 World Cup has been attributed to VAR catching fouls which would otherwise have remained unpunished. Of the 169 goals scored in the tournament, 22 were from the spot (with 29 being awarded in total), beating the previous record of 17 set in the 1998 FIFA World Cup. Jonathan Liew of The Independent compares the situation to the introduction of the Decision Review System in cricket and notes the changes it had on that sport, and suggests that it might lead to changes of a similar nature in football.
All of a sudden, VAR had been revealed to be just like any other replay system: a process meant to reduce the number of controversies by correcting human error was now only adding to the controversies because there was still human judgment involved. And no replay could render that judgment infallible.
The confederation says it wants to know whether the plays were reviewed in any way, saying "transparency is of essence."