Law 2 substitution and runners, batsman of fielder leaving the field, batsman retiring, batsman commencing innings

1. Substitutes and runners
(a) If the umpires are satisfied that a player has been injured or become ill after the nomination of the players, they shall allow that player to have
(i) a substitute acting instead of him in the field.
(ii) a runner when batting.
Any injury or illness that occurs at any time after the nomination of the players until the conclusion of the match shall be allowable, irrespective of whether play is in progress or not.
(b) The umpires shall have discretion, for other wholly acceptable reasons, to allow a substitute for a fielder, or a runner for a batsman, at the start of the match or at any subsequent time.
(c) A player wishing to change his shirt, boots, etc. must leave the field to do so. No substitute shall be allowed for him.

2. Objection to substitutes
The opposing captain shall have no right of objection to any player acting as a substitute on the field, nor as to where the substitute shall field. However, no substitute shall act as wicket-keeper. See 3 below.

3. Restrictions on the role of substitutes
A substitute shall not be allowed to bat or bowl nor to act as wicket-keeper or as captain on the field of play.

4. A player for whom a substitute has acted
A player is allowed to bat, bowl or field even though a substitute has previously acted for him.

5. Fielder absent or leaving the field
If a fielder fails to take the field with his side at the start of the match or at any later time, or leaves the field during a session of play,
(a) the umpire shall be informed of the reason for his absence.
(b) he shall not thereafter come on to the field during a session of play without the consent of the umpire. See 6 below. The umpire shall give such consent as soon as is practicable.
(c) if he is absent for 15 minutes or longer, he shall not be permitted to bowl thereafter, subject to (i), (ii) or (iii) below, until he has been on the field for at least that length of playing time for which he was absent.
(i) Absence or penalty for time absent shall not be carried over into a new day's play.
(ii) If, in the case of a follow-on or forfeiture, a side fields for two consecutive innings, this restriction shall, subject to (i) above, continue as necessary into the second innings but shall not otherwise be carried over into a new innings.
(iii) The time lost for an unscheduled break in play shall be counted as time on the field for any fielder who comes on to the field at the resumption of play. See Law 15.1 (An interval).

6. Player returning without permission
If a player comes on to the field of play in contravention of 5(b) above and comes into contact with the ball while it is in play
(i) the ball shall immediately become dead and the umpire shall award 5 penalty runs to the batting side. See Law 42.17 (Penalty runs).
(ii) the umpire shall inform the other umpire, the captain of the fielding side, the batsmen and, as soon as practicable, the captain of the batting side of the reason for this action.
(iii) the umpires together shall report the occurrence as soon as possible to the Executive of the fielding side and any Governing Body responsible for the match, who shall take such action as is considered appropriate against the captain and player concerned.

7. Runner
The player acting as a runner for a batsman shall be a member of the batting side and shall, if possible, have already batted in that innings. The runner shall wear external protective equipment equivalent to that worn by the batsman for whom he runs and shall carry a bat.

8. Transgression of the Laws by a batsman who has a runner
(a) A batsman's runner is subject to the Laws. He will be regarded as a batsman except where there are specific provisions for his role as a runner. See 7 above and Law 29.2 (Which is a batsman's ground).
(b) A batsman with a runner will suffer the penalty for any infringement of the Laws by his runner as though he had been himself responsible for the infringement. In particular he will be out if his runner is out under any of Laws 33 (Handled the ball), 37 (Obstructing the field) or 38 (Run out).
(c) When a batsman with a runner is striker he remains himself subject to the Laws and will be liable to the penalties that any infringement of them demands.
Additionally, if he is out of his ground when the wicket is put down at the wicket-keeper's end, he will be out in the circumstances of Law 38 (Run out) or Law 39 (Stumped) irrespective of the position of the non-striker or of the runner. If he is thus dismissed, runs completed by the runner and the other batsman before the dismissal shall not be scored. However, the penalty for a No ball or a Wide shall stand, together with any penalties to either side that may be awarded when the ball is dead. See Law 42.17 (Penalty runs).
(d) When a batsman with a runner is not the striker
(i) he remains subject to Laws 33 (Handled the ball) and 37 (Obstructing the field) but is otherwise out of the game.
(ii) he shall stand where directed by the striker's end umpire so as not to interfere with play.<
(iii) he will be liable, notwithstanding (i) above, to the penalty demanded by the Laws should he commit any act of unfair play.

9. Batsman leaving the field or retiring
A batsman may retire at any time during his innings. The umpires, before allowing play to proceed, shall be informed of the reason for a batsman retiring.
(a) If a batsman retires because of illness, injury or any other unavoidable cause, he is entitled to resume his innings subject to (c) below. If for any reason he does not do so, his innings is to be recorded as 'Retired - not out'.
(b) If a batsman retires for any reason other than as in (a) above, he may only resume his innings with the consent of the opposing captain. If for any reason he does not resume his innings it is to be recorded as 'Retired - out'.
(c) If after retiring a batsman resumes his innings, it shall be only at the fall of a wicket or the retirement of another batsman.

10. Commencement of a batsman's innings
Except at the start of a side's innings, a batsman shall be considered to have commenced his innings when he first steps on to the field of play, provided Time has not been called. The innings of the opening batsmen, and that of any new batsman at the resumption of play after a call of Time, shall commence at the call of Play.