Test cricket, unlike most team sports, does not have tactical substitutions.
The International Cricket Council (ICC), the sport's governing body, had
introduced a 10-month trial in July 2005
to use tactical substitutions for one-day internationals only.
But after much criticism from players, commentators and fans, the trial was abandoned in March 2006.
For all other forms of the game, from Test right through to village cricket, replacements can be used as a substitute fielder with permission from the umpires - while a runner for an injured batsman can also be used, except in international cricket.
A fielding team can use a substitute fielder if one of their team has been injured during the course of a match.
But they cannot be used in specialist positions, which means they cannot bat, bowl or keep wicket.
If the substitute fielder hangs onto a catch, it will go down as "caught sub" in the scorebook.
Substitute Gary Pratt memorably ran out Australia's Ricky Ponting in 2005
However, a sub cannot be used if a player wants to leave the pitch to change their equipment.
That player cannot come back onto the field of play without the consent of the umpire, otherwise the umpire can penalise the fielding team five runs.
If that player has been off the pitch for more than 15 minutes, they cannot bowl for at least that length of time they were off the pitch.
Test match regulations are slightly different as they come under the jurisdiction of the International Cricket Council (ICC).
Under Test match playing conditions, a player who has been absent from the field for more than eight minutes cannot bowl for the length of time they are absent.
They also cannot bat for at least that length of time for which they have been absent in the field or the start of their team's batting innings, unless their team have lost five wickets.
So if a player has been off the field for 20 minutes, they cannot bowl for at least 20 minutes from the point when they returned to the field.
However, these restrictions do not apply if the player has sustained an external blow (rather than an internal injury such as a pulled muscle) and has been forced to leave the field while playing the game.
If a batsman has injured themselves during the course of the game, they can use a runner to run for them between the wickets - except in international cricket.
If possible, the runner should be a batsman who has already batted in that innings.
If a batsman calls for a runner, there will be three batsmen on the field
The runner's protective equipment should mirror exactly what the injured player is wearing.
When the injured player is on strike, the runner will stand at square leg and run between the wickets from there.
And when the injured player is not on strike, the runner will stand at the non-striker's end.
The runner is subject to the same laws as the injured batsman, so they can be run out, out obstructing the field or handling the ball.
In England's domestic Twenty20 final in 2010, Hampshire's Dan Christian pulled a hamstring and called for a runner. As Christian's runner Jimmy Adams and batting partner Sean Ervine scrambled a match-winning leg bye off the final delivery, the injured batsman instinctively also ran through to complete the run.
He would have been run out if Somerset had put down the wicket at the striker's end as Christian was technically out of his ground - but the fielders' lack of knowledge of Law 2.8 (c) cost them the match.
It is possible for both batsmen to bat with runners - although the presence of four batsmen at the crease dramatically increases the possibility of a run-out.
In June 2011, the ICC board announced plans to ban the use of runners in international cricket, and this
came into effect on 1 October 2011.
Collingwood retired ill with a migraine during an ODI in 2010
A batsman can retire at any time during their innings, but the umpires must be notified of the reasons for retiring.
If a batsman retires hurt through injury or illness, they are entitled to resume their innings after a fall of a wicket.
However, if they retire hurt for another reason, they can only resume their innings with the consent of the opposition's captain.
If they do not resume their innings they will be recorded as 'retired - out'.
Sometimes, such as in warm-up games played by touring sides before Test matches, batsmen will retire after scoring 100 or 50 to give their team-mates batting practice - and in this instance, they are also recorded as 'retired - out'.