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A substitute in the sport of cricket is a replacement player that the umpires allow when a player has been injured or become ill after the nomination of the players at the start of the game. The rules for substitutes appear in Law 24 of the Laws of cricket.
The use of substitutes is known from the 18th century. In the report of a match on Monday, 5 September 1748, the role is termed a "Seeker-out"; this was in the sense that George Smith, who was carrying an injury and had been granted a substitute fielder in previous matches, was denied one in this match.
A substitute can act for the injured or ill player in the field, although he may not bowl, bat or as captain, unless otherwise agreed by the captains. A player may bat, bowl and field even if he has had a substitute for part of the game, though they need to wait for a period equal to their time off the field until they bat or bowl again. Substitutes are generally not listed in the official squad list, unless if they were in the starting XI for other games in the wider squad.
A substitute is permitted to take catches as with any other fielder, and on some occasions does. The first occasion in Test cricket was in the Test between England and Australia in 1884, when Australia's captain, Billy Murdoch, took a catch from his teammate Tup Scott while playing as a substitute for England. The highest number of catches by a substitute fielder in a Test match is four, a record shared by Gursharan Singh, Younis Khan, Virender Sehwag, and Jackson Bird. However, substitute fielders' catches do not count towards individual stats.
In recent years, there have been arguments made for cricket to allow substitutes in first-class games, as cricket remains unique amongst team sports in not permitting full substitutes for either injuries or tactical reasons. Arguments in favour have been made from a perspective of improving the game, coping with increasing injury rates due to the modern schedule, to follow the lead of other sports in trying to manage concussion more responsibly  and to provide greater opportunities for players to gain experience in first-class cricket. However, there is an equally strong viewpoint that the nature of the first-class contest may be diminished  with a summary suggesting that although medical experts would recommend introduction of substitutes the majority of players are still not in favour.
There have been calls for concussion substitutes with increased concussion awareness in the last 5 years. New Zealand has already introduced the rule for their domestic limited-overs competitions, having had two concussion-related substitutions in 2016.
Restrictions on injured fielder
When a player leaves the ground due to injury and is replaced by a substitute fielder, he/she is generally not permitted to return and immediately resume bowling (or batting if their team's innings commences while they are off the field). The injured player is required to spend a period back on the field at least equal to the time that they were absent before resuming bowling. Variations of the time periods required and the circumstances of the players return to the field apply in different forms of the game.
In 2005, the International Cricket Council announced, as part of a package of changes to the playing conditions for One Day Internationals to be trialled over a ten-month period, that tactical substitutions would be permitted. Each team was to be allowed one substitute, who had to be named before the toss was made, and could be introduced at any stage of the match. The ODI series between England and Australia in July saw the first use of these new regulations, which did not apply to other forms of cricket such as Test matches.
This change, however, was widely criticised by players, commentators, and fans. In particular, it was said to give the team that wins the toss an even greater advantage than usual.
In March 2006 players and officials started to rebel against this controversial rule and a One Day International series between South Africa and Australia saw the players agree to boycott the rule. Just a few weeks later the International Cricket Council announced that the rule was being withdrawn, and it is no longer used.
In the 2005 Ashes Test series, Australian captain Ricky Ponting repeatedly complained that England were abusing the substitute system by bringing on specialist fielders in place of bowlers with poor fielding skills, which he argued was against the spirit of cricket. English bowlers were frequently substituted at the end of bowling spells and temporarily replaced with fresh fielders. The England coach Duncan Fletcher argued that these substitutions were either legitimate injuries or players "answering the call of nature" (i.e. using the toilet). The issue came to a climax in the 4th Test, when Ponting was run out by substitute Gary Pratt, causing him to angrily shout and gesticulate in the direction of the England dressing room. At the time, Pratt was on the field for bowler Simon Jones - who was nursing an ankle injury that proved to end his Test career after this match. Ponting was fined 75% of his match fee for dissent.
In 2008 the International Cricket Council tightened the regulations on the use of substitutions, saying "Substitute fielders shall only be permitted in cases of injury, illness or other wholly acceptable reasons...and should not include what is commonly referred to as a 'comfort break'". 
If a bowler is injured during an over and cannot complete it, another bowler must bowl the remaining deliveries. The bowler chosen to do so cannot be the bowler who bowled the previous over, and must not bowl the following over either. A substitute fielder may take the place of the injured bowler whilst they are off the field, but they may not bowl.
- Ashley-Cooper, At the Sign of the Wicket, 12 April 1900, p. 52.
- "Scorecard Test No. 15". Cricinfo.com.
- "Most catches by a substitute in a match". ESPN Cricinfo. Retrieved 15 January 2017.
- "Australia team doctor asks ICC to consider concussion substitutes". Cricinfo. Retrieved 7 January 2017.
- "New Zealand to introduce concussion substitutes". Cricinfo. Retrieved 7 January 2017.
- "Archived copy". Archived from the original on 26 December 2008. Retrieved 1 June 2009.CS1 maint: Archived copy as title (link) ICC rules and regulations
- "Cricket bosses explain sub trial". BBC Sport. 11 December 2005.
- "Ponting slams England sub policy". BBC Sport. 29 August 2005.
- "Ponting set to take comfort from ICC new fielding rules". The Sydney Morning Herald. 4 July 2008.