Law 27 (Appeals)
1. Umpire not to give batsman out without an appeal
Neither umpire shall give a batsman out, even though he may be out under the Laws, unless appealed to by the fielding side. This shall not debar a batsman who is out under any of the Laws from leaving his wicket without an appeal having been made. Note, however, the provisions of 7 below.

2. Batsman dismissed
A batsman is dismissed if
either (a) he is given out by an umpire, on appeal

or (b) he is out under any of the Laws and leaves his wicket as in 1 above.

3. Timing of appeals
For an appeal to be valid it must be made before the bowler begins his run up or, if he has no run up, his bowling action to deliver the next ball, and before Time has been called.
The call of Over does not invalidate an appeal made prior to the start of the following over provided Time has not been called. See Laws 16.2 (Call of Time) and 22.2 (Start of an over).

4. Appeal "How's That?"
An appeal "How's That?" covers all ways of being out.

5. Answering appeals
The umpire at the bowler's end shall answer all appeals except those arising out of any of Laws 35 (Hit wicket), 39 (Stumped) or 38 (Run out) when this occurs at the striker's wicket. A decision Not out by one umpire shall not prevent the other umpire from giving a decision, provided that each is considering only matters within his jurisdiction.
When a batsman has been given Not out, either umpire may, within his jurisdiction, answer a further appeal provided that it is made in accordance with 3 above.

6. Consultation by umpires
Each umpire shall answer appeals on matters within his own jurisdiction. If an umpire is doubtful about any point that the other umpire may have been in a better position to see, he shall consult the latter on this point of fact and shall then give his decision. If, after consultation, there is still doubt remaining the decision shall be Not out.

7. Batsman leaving his wicket under a misapprehension
An umpire shall intervene if satisfied that a batsman, not having been given out, has left his wicket under a misapprehension that he is out. The umpire intervening shall call and signal Dead ball to prevent any further action by the fielding side and shall recall the batsman.

8. Withdrawal of an appeal
The captain of the fielding side may withdraw an appeal only with the consent of the umpire within whose jurisdiction the appeal falls and before the outgoing batsman has left the field of play. If such consent is given the umpire concerned shall, if applicable, revoke his decision and recall the batsman.

9. Umpire's decision
An umpire may alter his decision provided that such alteration is made promptly. This apart, an umpire's decision, once made, is final.