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 all appear in Law 2 of the Laws of cricket.
A substitute can act for the injured or ill player in the field, although he may not bowl, bat, or act as wicket-keeper or as captain, unless otherwise agreed by the captains. A famous example is the 1986 Test between England and New Zealand at Lords, where England used 4 different wicketkeepers due to injury to their original wicketkeeper. (But see "Tactical substitute" below.). A player may bat, bowl and field even if he has had a substitute for part of the game.
A substitute is permitted to take catches as with any other player, 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.
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.
In the SCG test in January 2017, Matthew Wade had been out with illness, with Peter Handscomb his deputy, while Matthew Renshaw was ruled out at short leg after being hit on the helmet while fielding and had also been hit in the jaw as a batsman before, but when Peter Handscomb also got hit in the groin, Jackson Bird also deputised for him. With Renshaw being ruled out, there had 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 subtitutions in 2016. However, substitute fielders' catches do not count towards individual stats. A recent example is when Tim Southee was dropped for Jeetan Patel in the 1st Test against South Africa, and Southee went on to take a catch to remove Hashim Amla after Ross Taylor was injured and was unable to field.
Retired hurt (or not out)
If a batsman is injured or falls ill while batting, he may retire and resume his innings at the dismissal or retirement of another batsman. If he cannot return by the end of the innings, the batting side must close its innings after all other batsmen are dismissed (excluding the not-out batsman). This can occur if the batsman requires medical attention away from the ground. It is therefore possible for the side batting last in a match to lose despite only losing nine (or potentially fewer) wickets. For the purposes of calculating a batting average, a scorecard entry of 'Retired, hurt' or 'Retired, ill' is considered not out. Most batsman will return as the no.11 batsman to give them as much time as possible to recover sufficiently to continue their innings.
In cricket, a batsman retires out if he retires without the umpire's permission, and does not have the permission of the opposing captain to resume his innings. This occasionally happens in friendly or practice matches, for instance English county sides against University Centres of Cricketing Excellence. Although it is not considered to be a dismissal in the context of a cricket match, it is considered a dismissal for the purposes of calculating a batting average.
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 football-style 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 NatWest Challenge 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 Ashes test series of 2005, Ricky Ponting complained about what he regarded as the tactical substitution of specialist fielders for weaker bowlers in the English team, which he argued was against the spirit of cricket. English bowlers were frequently substituted at the end of bowling spells and replaced with fresh fielders. The English coach argued that these substitutions were either legitimate injuries or players "answering the call of nature". Ponting was fined 75% of his match fee for dissent after being run out by a substitute fielder during the fourth test. 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'" 
- "Scorecard Test No. 1049". Cricinfo.com.
- "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 2017-01-07.
- "New Zealand to introduce concussion substitutes". Cricinfo. Retrieved 2017-01-07.
- Law 2, 9(a,c)
- Cricinfo scorecard 2nd Test Sri Lanka vs Bangladesh at Colombo 6–10 September 2001
-  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.